I slapped together a S40 B000 transmission last night and found that the interlock (part number 24430-PL3-010), is not the same between the S40 and S20 transmissions despite both having the same part number.

#7 in picture below



The S40 interlock has a steeper angle on one side so that it can clear the casing. The S20 interlock is not compatible with the S40 transmissions, but it's more than likely that the S40 interlock is compatible with the older S20 transmissions and that the current part number listed for both is for the newer S40's interlock and supercedes the S20's older part number.

 

If a new part is listed for an older trans, it does in fact supercede the old part.

 

I could only guess that it was a newer part superceding an older part, but it could also be that the older part was used with the newer transmission. I doubt the latter because the two pieces are definitely different.

How I discovered the issue:
When I disassembled the transmissions, I just threw all of the shift interlock and shift rod pieces into a bucket thinking that all B000 parts were the same except for the obvious pieces. I assembled the trans all the way to gear housing and when I tried to get it into the 1st-2nd gear gate, I could get the interlock piece far enough over to get the lever in place to kick 1st or 2nd into gear. I compared three interlock pieces that I had and only one was different (and I only had one S40 B000).