I forgot that I posted this. I am still looking for this part, I could get it directly form IRD but the part costs six bucks and there is a minimum order amount that would require me to buy seven of 'em to get just the one... I'll post a picture in case anyone has something similar. I got the bike working by using a seatpost shim and the seatpost/saddle from my other bike, but it is annoying to have two half bikes...
In the pic, it is the part that the seat rails would sit on, the lower clamp piece. If you have something, please drop me a line - that, or if you need something form IRD, maybe we could get together to exceed the minimum order quantity.