Lou,
The 1396560 and 1396561 pump numbers look to be the latest revisions for 1957-58 Buicks. These two numbers were available at the dealer as late as ‘72 and provided universal coverage regardless of fan size or AC usage. Earlier pump numbers were superseded by these two. See attached page from the ‘72 Buick Master Parts Book
The 1396560 is for:
1957 All
1958 Without AC
1958 With in-dash AC (first type pump)
The 1396561 is for:
1958 With in-dash AC (second type pump)
The changeover occurred at Engine Mfg. No. 456352 for Limiteds. I was not able to find out what the differences are between these two pumps. There were a bunch of other mid-year updates for cooling system components. Data on when the changeovers occurred was either not available or did not match the 456352 breakpoint.
Cars with AC in these two years used 5-blade fans except for some late ‘58s which used a 4-blade fan that continued on into ‘59. So the factory application data shows the 1396560 pump was used on many cars with 5-blade fans. This conflicts with the application data from modern-day pump suppliers. They all get their application data from the same place, as once an error creeps in, they all show it.
Buick dealers offered an under-dash AC unit called the Kool-Pak in 1958. The 5-blade fan on your car could have been installed when a Kool-Pak was added. Or it could have been ordered as part of a heavy-duty cooling option to accommodate future AC installation. If the original GM Frigidaire Kool-Pak compressor is still in place, it will have a single drive belt. Compressors for in-dash systems used two belts.
After all that, rebuilding the original pump may be the easiest route.
