The letters (XLM, QLM, VLM, K8, ZLM) were different ADC cartridge models... but the interesting thing is that the bodies manufactured were always identical - all that varied is the stylus. (there are a couple of exceptions, but...)
I've had both the silver and gold colored ADC's open (the adhesive gave up) and pulled the case body/sleeves off and I found they were quite different. I've also heard the silver ones were slightly less desirable and were used with the lower-end cartridges like the QLM, VLM, K8 etc. and the gold were used with the higher-end better cut styli like the ADC XT IV(nude diamond) ZLM and most of the XLM variance's.
Of course I also have several 'XLM II' with a black colored case/sleeve. Exceptions; there surely was.