As I mentioned earlier get the serial number and check to see if it really is a post '64 Model 88. The seller might be mistaken.
If it is a post '64,then its a replacement stock or at the very least a 'found' early stock if the serial number indicates a Model 88 made at the end of the production run. Could be that Winchester had leftover stocks to use up.
Replacement after market stocks were made (by Boyds I think). I have seen an ad for such on the EE. But that was some time ago.
The 88 cracks at the magwell because the wood is thin there. It also cracks at the wrist on the early pre '64 88s that have the 'four leaf clover' receiver. From his pics the seller's 88 doesn't seem to have that type of receiver.