Take this all with a grain of salt and for sure, verify what you have as you are planning to do with the Mityvac...
I found a GM-to-Delco cross-reference online here:
Link to Delco Cross Reference
It says your 1973624 is a Delco D1369A
I then found a reference
here (credibility?) that says the NAPA/Echlin equivalent to a Delco D1369A is VC1859.
Finally, in the Lars Grimsrud "Distributor Vacuum Advance Control units
Specs and facts for GM Distributors" doc that is widely quoted online, the VC 1859 specs are listed as:
Starts 3-6" Hg
Max of 10 degrees (20 crank as you state) @ 9-12" Hg
If you have an original Delco part, it may well match. If aftermarket, I recently found a Std Motors advance unit was close on both the start and the max specs, but there was no mechanical limit at the max, so the advance continued beyond the max-rated vacuum level.
Good luck!
Mike