I found a part number for that weird rear turn signal bulb on the 2nd gen Venture, and, I suppose, the RSTD also.
7507
Also referred to as the BAU15s on this website:
http://autolumination.com/1156_1157.htm
You can see these bulbs down near the bottom of the page.
They also have LED replacements, if you want to actually DO that.
I got the impression during my searches that this bulb is used in some European cars, like the BMW and the Porsche. How it ended up in our bikes is anybodys guess, maybe the Japanese designers got a helluva deal on the bulbs and sockets.
Someone somewhere has asked if there were any bulb numbers for this bike, and this was the only one I didnt have till now.
Hope this is of some use.