The surcoat Magnus wears is a simple garment constructed of a front and back piece of linen fabric, sewed together and lined in linen. The ornamentation is applied in a flannel wool, which due to it's felted nature, resists fraying. The garment is designed to go on over the head, and has side slits as to not inhibit his range of motion.

In Arms & Armour of the Medieval Knight: An Illustrated History of Weaponry in the Middle Ages by David Edge and John Miles Paddock, the authors note on page 45 that "...surcoats, long, usually sleeveless, full-skirted cloth gowns which the knight wore over the armour....first appear in the second half of the twelfth century, but did not become widely popular until the thirteenth century." Pictoral and sculptural evidence for the surcoat is vast.

Twelfth and thirteenth century examples show a surcoat which is long in both the front and the back. By the early to mid 14th century, many examples of the surcoat show a shortened front. The small image to the right is a detail of a fresco by Maso di banco, c.1340 in the Church of Santa Croce, Florence. (Original image on wikipedia in the public domain.) The coat of plates beneath the surcoat is also clearly visible.

By mid to late century the surcoat was seen predominantly short in both the front and the back. On page 45 of Arms & Armour of the Medieval Knight Edge and Paddock note that "By the late 1340s [the surcoat] had risen to knee level both at the front and at the back." This garment was sometimes depicted as form-fitting over the torso and it was not unusual to see front or side lacing. Two examples from the 1340s appear below.

During this period, pictoral examples of the surcoat can be found with the length varying knee length to hip length. In the detail from the image of King Laszlo, c. 1360, the king appears to be wearing a surcoat which is hip length front and back. He does not appear to be wearing a coat of plates underneath, but does have on a mail hauberk.

All that being said, Magnus's surcoat would probably be more accurate for his mid-14th century portrayal if either the length of the front and back were extended to mid-thigh or knee length, or if the front of the surcoat came up just a bit to reveal the coat of plates beneath and the back extended to knee level.

The ornamentation on Magnus' surcoat is a livery meant to identify him as a member of our SCA household. The use of such liveries and heraldry is asserted to have been brought over from the East by the Crusaders. In 20,000 Years of Fashion: The History of Costume and Personal Adornment: Expanded Edition by Francois Boucher, the author states on page 188 that "The most curiuos practice borrowed by Crusaders form the East was tht of having their arms painted on their shileds, originally to rally round their men in battle, then later as a fashion, perpetuated by tournaments."

There is ample evidence for male heraldic surcoats—both pictoral and extant. The image to the left is a coloured illustration of a monumental brass of Sir Hugh Hastyngs dated 1347. The heraldry on his shield is also displayed on his surcoat. The image on the right is of King Edward III and is dated circa 1340. It is the jupon or surcoat of the Black Prince which is one of the period heraldic garments which survived, a reconstruction of which hangs over his tomb at Canterbury Cathedral.