I dont think rear drag reels can have spacers/shims for their spools as a shim would stop the spool locating onto the "peg" on the shaft.I think Im missing a shim inside the reel.
I've yet to see a Shimano rear drag reel supplied with spacing washers. I suspect you are right about them interfering with the spool locking if fitted.
However, I have encountered the same issue on a similar vintage Shimano Baitrunner reel. It transpired that two supposedly identical spools, weren't. One had a fractionally longer skirt. It manifested in a fractional catch on the rotor at the lowest point of travel in the spools oscillation cycle. The only solution I found was to not use the spools that caused problems.
Not one of my reels (Drennen, Diawa and Shimano)have come with any washers! Should they?
Depends on the reel type and model. The little shim washers, that are sometimes supplied with front drag reels, are not for making spools fit. They are to adjust the front to rear line loading balance. The vast majority of reels give a nice even level straight out of the box. If a new reel doesn't load level fresh from the box, I'd suggest that you take it back to the shop.
Though there are reasons to do it, the vast majority of coarse anglers will never have any need to bias line loading to the front or rear of their spools.