I have suspected that piece of foam was to help isolate the tank from vibration. The '68 tank has that funny "U" bracket underneath, with the single 6mm bolt, which they changed pretty early. I always wondered if that bracket did not quite 'cut the mustard' for them, leading to the much more solid setup with the rubber bungs and cup setup they used from then on. I would keep it, or replace it with new. Cannot hurt the situation.

1971's were the first year for turn signals, most are missing on bikes. That's why I passed on a 1970 HT-1 and bought a purple 1971 purple HT-1B...to get the turn signals and a purple bike.