A few months ago I was cruising along I-eBay and noticed a bulk lot of used U.S. definitives closing in three minutes or so. There were no bids, so I threw in a bid for a buck or two and won. I've got about $6.50 in the deal including shipping. Don't ask me why I bid - I don't know. I was there. The lot was there, We were both lonely. Wait . . . that's a different story.
Anyway - I haven't even looked at the lot so there could be that missing C3a in there for all I know.
The lot was made up of numerous envelopes holding 100 or so of the same issue in each. There's probably a densely packed shoebox of them. Someone willing to send me anything interesting in the $10 catalog range can get these shipped for your group. Just shoot me an email.