I'd just like to add something slightly relevant that I also posted in a different thread a while ago...

When Apple switched from caddy-loading to tray-loading CD-ROM drives, they continued selling the same models with the same names; they just started making them with tray-loading drives instead. It was indicated with a /B rather than /A model number:

http://tidbits.com/article/2242

So some models with CD drives came stock with either a caddy or a tray drive, depending on when they were manufactured. I'd assume this would apply to all/most Mac models that were in production in the early 1994-ish period of time.