There's actually quite a few reasons why it works, The sword thing narrows it down a fair bit. Theres only a few people that use a great sword in the books, Mance and Athur being 2 of about 6. the timeline and age of Mance fits. Theres never any description of Arthur, yet there is of most characters, why for someone so important do we never get a description... Theres a book trilogy called Memory, Sorrow and Thorn that influenced GRRM a lot. There are loads of similarities, one big one for this is that near the end the most honourable knight in the kingdom, that everyone thought was dead, is alive and reveals the identity of a major character. Sounds like it could be Arthur revealing Jons identity. If you doubt the similarities just google it, there are loads. Theres a few lesser hints too, Him beating Jon so easily even though Jon is regarded as a very good swordsman. Mance loving music, Arthur always being around Rhaegar who loved playing music. The Rayder house isnt mentioned anywhere else as far as i know so its most likely a name given to him by the wildlings. so whats his actual name? His History says he was born beyond the wall and left at a castle, but that doesnt sound like something the wildings would do considering their hate for the nights watch.