many thx to everyone who replied to my query...
as of yet, there doesn't seem to be a definitive answer to my original question...however, i'm starting to think that ANY (or most) of the tray-loading cd-roms will be compatible with the CR-173 drive i wish to replace (including model 678-0178, which Ron didn't mention).
The following reply came from tech support @MicroDocUSA (a company that sells Mac replacement parts):
"Many years ago, the rumor was, that you could only replace the CR173 with a CR173. That has actually turned out to be incorrect. The only tray load optical drives we have available right now are CR176 or CR177 and we have shipped thousands of them to schools around the country without any reported problems".
Tech support
MicroDocUSA
I then re-checked with this same person:
"Just to be absolutely sure that I understand this correctly...are u saying that I can replace my CR173 with any of the following models: 175, 176, 177?"
Reply: "That is correct!"
As such, the issue is still a bit fuzzy...and I'd be grateful for any further insights.
Best wishes to all!