Good morning:
S and S Firearms, Glendale, NY (must use Glendale in Internet search) shows center-fire Spencer blocks in stock.
S and S prefers inquiries be via email, not telephone.
S and S may be preparing to be at the N-SSA Nationals next week, so replies could be in mid-October.
There have been a few instances of center-fire blocks not easily fitting due to minor variations in original receivers, so before ordering, please measure the original rim-fire blocks and compare them to the centerfire version at S and S. Smiles.
El Supremo/Kevin Tinny