The "S" in IR400S denotes a synthetic base. I'm thinking that the Retro 400S and the IR400S and the IR400 are all the same film. Incidentally, I'm told that the Rollei Retro 80s also has a bit of IR sensitivity - out to 750nm, where I think the IR400 goes out to 820nm before the sensitivity drops to zero.

I got a slide of the IR negative through a hybrid process which is verboten on here. DR5chrome can reversal process this film, but I haven't had the nerve to send it to them yet as they're saying they only see partial IR effects, where it looks like I'm getting full IR effects with my film by using a cheap Chinese 720nm filter and sending it through a non-reversal process.