We used neutral buffered formalin (Lillie's formulation) in early
60's.  One of the problems is anything fixed Bouins or picric acid based
fixatives, alcohol based fixatives or heavy metal fixative.
these are probably not going to be suitable for PCR technics.
Hope your records go back that far, I have some that go back to 1913!
in a vet research facility if some well meaning cleaner-upper has not
trashed.

Remember the great flu epidemic sleuthing done recently.  They went into
AFIP archives, pulled tissues fixed in formalin from a soldier who
died of that flu strain, did PCR and found it was a swine flu variety with
origins in the USA and not the imfamous 1919 (?) "Spanish flu" misnomer.

Hope I have my facts straight on that one, but it is a great story, plus
giving us a wealth of archived information, NBF fixed!

Gayle Callis