How does one identify, externally, a 999 from a 904?

Man, other than possibly the serial number, I think they are identical on the outside. Knowing what kind of car it comes out of can give a clue, but other than that, I don't know. Maybe others will chime in here that know much more than I have in my limited brain. I'm working on a 999 now - it came out of a police car. Sorry I couldn't be more help.

Dave

The trans can be identified by its part number which is stamped in the drivers side pan rail.

John, can you be a bit more specific? Is there a designation/digit that will ID the unit?

The first seven digits is the part number, it is often preceded by a PK or single letter.
Check this link for the application:

http://www.mymopar.com/downloads/transguide.pdf

The following are part #'s for A999 trans.(non lock-up) These #'s are from the direct-connection chassis book.

'74-'75 3681844
'76-'77 4028465
'78 4028878

Not to muddy the waters, but weren't all 998's and 999's lockups?

I was told one id'er is if the input shaft was hollow it was a lockup.

The A998 and the A999 were both introduced in '74, the lockup converter wasn't introduced until '78.