Are U.S. Belts different to U.K. ones?
I use the fleabay extenders and they seem fine, under £15 a pair.
I'll have a look at the belt tongues next time I'm the garage and compare to trad and tintop.

1969 4/4
1995 plus 8
1996 Plus 4 4seater
2002 Aero S1
2013 M3W
2014 Plus 8