Dang, I can't find my list either!
But I've got the list of plate numbers for all the '87s and 422934 is on there so your '88 has to have had 432934. I'm pretty sure the '88s got tags in around the 4328XX-4330XX range but I'll have to find the list to confirm that.
The in-service date labels were generally applied somewhere on the lower left portion of the dash on most vehicles, including the Mustangs up to '86. Here's one from an '82:
Once the dash design changed with the '87s, they moved the label location to the left side of the dash, the portion that is up against the door panel. Here's a shot of a '93 for sale taken on the CHP lot:
I found glue residue in that location on my car but the label was long gone so, for my car at least, either the CHP removed it prior to sale or most likely it just fell off at some point during the nearly 12 years they had it. I'll bet it's the same case for your '88, it either fell off or was removed, most likely by a previous owner.