It was the only one converted from a UK Mark 3.
As 'josefstadt' said, 6401 was part of the International set and is significantly different to a standard Mark 3, with basically only the interior the same as a standard Mark 3. Different bodyshell, construction method, doors, bogies - the lot. Whereas 6402 has standard BT22 bogies (as shared with 6105 and 7146, both preserved) and also worked with Mark 3 Push/Pull sets, something which 6401 never did.
6402 in service with a Push/Pull rake -