Your comment about modified road bikes (big tanks, big hubs, real seats on real rear loops and not really too handy looking) would also apply to most British trials bikes GOV132 excepted that were riding around Pre65.
The Montesa i saw competing looked incredably similar to an early 247 Cota as quite a few commented. Sounded very sorted too. Thing was, i've no idea either, who actually knows exactly what a pre65 Montesa should look like. At least we have photographic evidence of the Bultacos that were competing in the UK Pre65. Then again how many Arials competing today look how they did in 1964 ?? Not many.
Sorry about my earlier comment about the word British being included in the regs i was obviously mistaken.