I have never encountered the RP3105, so take this for what it is worth.
Still, based on other Riso model numbers, I'd guess that the RP3700 drums might be physically wider than drums for the RP3105. Check the print area specs of the RP3105 first though to make sure I'm not full of crap, though. If that machine can print 11" X 17" with the drum that is in the machine, I am mistaken, sorry.
On a lighter note, you might want to try here for Riso help:
http://www.copytechnet.com/forums/digital-duplicators/
There are more people there with Riso experience.
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