As for the spacer, I asked the same question when it arrived to me.
I guess many of the GBR ones do, as my mech buddy said he's seen most GBR D90 ones like that. It may be for length adjustment or may be for angles from the diff drive flange, I'm not really sure.
The spacer is machined with a centering portion in the middle to ensure axial alignment and balance and is threaded for those four bolts shown in the photos.
This is a rear one, and I'm not sure about your rear differential, etc., but if you have a standard four-bolt drive flange on your rear diff (like the 90s/Disco Is/RRCs from +/- 1990ish on certainly have) and your transfer case is an LT230 and standard driveline/drum brake flange behind your LT77, I expect it should work fine. As I recall, it is slighty longer than stock to allow for the taller lift I had on the truck before.
Hope this helps! Maybe someone here who has more experience with ROW/80s Ds can chime in?
Thanks for your interest,