Iconographic evidence is spotty, since body armor is very often covered by a jupon or cote-armure. When it isn’t, rear views are frustratingly rare. Brasses and low relief effigies usually show only the front of the harness. Surviving plate body armor from the period is very rare.
That said, there is some visual evidence for plate back protection of small to medium size plates in the late 14th century.
One piece backplates, however, seem to have been a relatively late development. The earliest illustration of a one piece backplate that Claude Blair was able to trace was the brass of John Ruggewyn (d. 1412) in Standon, Hertfordshire. The effigy of John de Montacute, d. 1400, shows a row of hinges down the right side of his jupon, suggesting a backplate.
When there is a back defense, the body armor can open in several ways. It can open in front, as shown in Altichiero’s Execution of St. George and Battle of Clavijo. These, however, are all infantry armor. Armor that opens in front is easy to put on without assistance, but creates a point of weakness that is exposed to the enemy.
Men at arms, who could afford servants to assist them, seem to have preferred armor that did not open in the front, although a Spanish St. Michael from 1400-25 shows the angel in the leg harness of a man-at-arms and front-opening body armor. One of the front-opening breastplates from Chalcis was equipped with staples for the lance rest of a man at arms. Another originally had holes where a lance rest would have been attached.
A simpler body armor from Chalcis, made of rectangular plates is also preserved at the Met. Aside from using larger plates it closely resembles a similar tubular, front opening body armor buried after the battle of Wisby in 1361. Since even infantry armor was typically shown with a distinct waist in artwork from around 1380 on, I suspect this armor was either made before 1360 or represented a very cheap and low grade product. Note that the lower plates of the line drawing are a speculative reconstruction: see the photo in the same thread for what actually remains.
I should mention that the velvet covered body armor prominently displayed in the Met is an unreliable reconstruction.
An opening at the center back is much less vulnerable, and the Prague St. George is a good example of this sort of design.
The Lincoln misericord shows another approach. One plausible reconstruction is that the main body armor wraps ¾ of the way around the body, overlapping a back defense of smaller plates.
Churburg #13 wraps ¾ of the way around the body.
Finally, the body armor can open at the sides. The Munich fabric covered breastplate probably once had a back defense that attached to hinge plates at the shoulders, one of which survives. A waist belt may have kept the front and back together as on the Churburg #18 cuirass from around 1420.
Or both sides could have been closed with laces, as on many later brigandines. Here is a group of photos of the Munich harness. Note the excess fabric on the wearer's right. There might originally have been eyelets or rings for lacing the sides closed that have not survived.
Alternatively, it may have opened only on one side. The side opening coats of plates from Wisby all open on one side: two on the left and one on the right.
For a body armor with a close neck and plates lining the upper shoulder straps, at least one opening at the shoulder is essential for side-opening armor and highly desirable for center openings. My first coat of plates for this period opened at center front but not the shoulders, and over time several of the rivets at the shoulders tore through the leather covering from the stresses of putting it on and off. Being able to unfasten both shoulders simplifies storage considerably, particularly for side-opening armor.
A body armor at the Castello Sforzesco in Milan, dated by the museum to 1380-1410, laces at both sides and shoulders. The museum calls it a corazzina, but an Englishman might well have called it a pair of plates. Here is more on the construction details.
While this armor and many later brigandines lace at the shoulders, hinges with removable pins as on the Munich covered breastplate might be less vulnerable to penetration and damage.
While the Milan armor is somewhat unusual in combining plates and mail beneath a fabric covering, it is not unique. A late 16th century brigandine in Philadelphia similarly has mail at the shoulders, skirt and sides. The first two would provide flexibility for shoulder movement and sitting, and the mail at the sides would help keep the plates of the front and back from fouling each other.
Dmitry Nelson has posted several pictures of surviving plates that may be from this period or somewhat later. The asymmetrical plates protect the right or left side of the chest, and I believe the others were all part of back defenses. The plates from Paris are inverted.
Here are two dorsal plates from Chalcis in the Met. One of them was also shown in an earlier plate Dmitry Nelson reproduced above, from when it was in Athens.
Here is more detail on the finds at Otepaa castle in Estonia. Note that what the article labels as small breastplates are almost certainly dorsal plates shown upside down. Also, more recent scholarship has cast doubts on the 1396 date for the destruction of the castle, suggesting that it might have been destroyed some time later but before 1420.
Guiron le Courtoise 1370-1380 Body armors with regular horizontal rows of rivets front and back, combined with a tailored outline, rounded breast and narrow waist.
Speculum Humanae Salvationis 1370-1380 Body armor closed with a row of buckles at center back.
This 1387 illumination shows men at arms wearing body armor, and the backs that are visible show a regular pattern of rivets.
Spinello Aretino's 1407-08 fresco depicting a 12th century naval battle shows back views of a variety of body armors.
I think there's some deliberate archaicism in the fresco: some of the combatants are wearing great helms, which I have not seen in contemporary depictions of more recent battles.