I am not at all sure what it shows. This is partly why I ask if anyone else is "considering" the possibilities.

The small amount of surviving clothing (surcoat for combat often being called "sideless" in the upper body region) looks pretty different in the upper body region than much of what is shown in the Morgan and Maciejowski illuminated transcripts. I have not seen a surviving example with the side slits near legs that the illuminated manuscripts frequently depict when showing them in mounted context. The slits on the sides of those surcoat illustrations strike me as "inventions" to make them comfortable for mounted use. It is also a good possibility that there simply is no surviving example of the simple functional "cavalry style surcoat" from that era.

At least in Chauser's Prologue, he describes the knights' surcoat as; "a long surcoate that reached to his knees".... The story departure location where the prologue begins was the "Tabard Inn." It has left me thinking that a mixture of styles could generically have been called surcoats in military context.