I've tried finding the rules for the different organization rules, FEC site is typical government jargon like the tax code!
I did find info on a liberal site, basically how they skirt the law. This is from bushout.tv : (the typos are theirs, not mine I'd fix it but after going to the site, need to take a shower ASAP!)
MoveOn.org (501(c)(4) educational organization) is a grassroots advocacy group can loby for legislation. It is legally separate from the MoveOn PAC and the MoveOn Voter Fund.
MoveOn PAC (PAC, affiliated with MoveOn). The MoveOn PAC primarily funnels grassroots money to Democratic candidates in close races.
MoveOn Voter Fund (527, affiliated with MoveOn). The MoveOn Voter Fund's strategy is to play anti-Bush ads in swing states early and often. The Voter Fund has raised nearly $10 million from its members, along with $5 million from George Soros and Peter Lewis.
This does show you how these orgs are set up to cover every base.
The next step would be to find someone who has the patience to read the FEC legal stuff, and find out if the MoveOn PAC is allowed to borrow envelopes from any of the other MoveOn entities. Asking the FEC to investigate it is only going to get you a lot of waiting.