Looking at the 1978 and 1979 Radio Shack catalogs, it appears the cartridge that came with the LAB-400 was the R-1000ED, a bright red clone of Shure's M91ED.
The R8X stylus was meant for a later Radio Shack cartridge, the sonic equivalent of Shure's M95ED, but it will fit and work in the R-1000ED. It's not a bad stylus, though it's "only" a 2x7 elliptical and only a slight step up from the M91ED's stylus.
Many decent styli can fit the R-1000ED/M91ED, but it sounds best with its own stylus. In Radio Shack speak, this was the RS8E. In Shure-speak this was the N91ED, good and inexpensive aftermarket versions of which are still available from a distributor called EVG. Do a search for EVG and M91ED in this forum (or on eBay) and you should find some encouraging suggestions.