1996 Aprilia RS250 has Mikuni TM34 flat slide carbs: http://www.motorcycle.com/mo/mcaprilia/rs250.html
Some other info:
http://www.procycle.us/main/mikuni_dimensions.htm
http://www.cyclewareables.com/pages/mikuni_tm_series/mikuni_tm_series.htm
Hope you find what you're after.
Cheers.
_________________
Jerry Lai
NSR250 [MC21SP]