According to Lotus Supplies (Banks) the wider seal fits the 336 and the narrower the 352/365.
According to RD Enterprises lists it the other way round! Wide = late, narrow = early.
If you are going to use your original diff nuts, then take one out, after carefully marking its position, count the turns and mark the release point and check which seal is correct.
If you are changing your diff nuts, then the ones for the R16 4 speed will work.
OK, now the bad news, if you are changing your diff nuts, you need to pull the tranny and remove the bell housing so you can set up the diff freeplay and preload properly.