The R1 would have been my first spot, and then you posted some pix of that one, which I guess says that wasn't your virginal droid custom.
I also saw some R5 dome swaps, but that would be too recent. Honestly, alot of R5s would qualify.
The Dagobah R2 comes to mind, since the body is done, but the legs aren't. I wish that I could blow up the picture.
I'm not familiar with the R2 next to the custom R1, but that may be a comic pack droid, or Wal Mart exclusive that I haven't seen. There are a couple of R2 units above the POTF2 R5s that I don't recognize, but my final guess would be:
The POTF2 R5 next to the redo R5, and also next to the original POTF2 R5. It looks like you did a much improved leg swap.
That would be my pick of your first astromech mod.
Barry