Ironically I suspect that there may be more surviving armorials from the late medieval period showing the heraldry attributed to King Arthur and his knights, than for almost any other non-royal historical figure from this era.
It is crazy that we have far more near contemporary sources for the heraldry attributed to obscure fictional knights from the medieval romances, than we do for the uniforms of many of the regiments at major battles like Blenheim fought several hundred years more recently.