Ok...I see what is going on, eBay UK has the manuals listed with British prices but they are shipped from US vendors.....sneaky.
Here is some info from the Resources section of the forum with an engine table:
https://p15-d24.com/page/p15d24/reference/reference_information.html/
Looks like the '42 and '50 Plymouths both had 218 engines. Compression ratios may be slightly different, there are knowledgeable folks in here that can enlighten us.
My manual says '48 (which I think is same as '42) had 6.6:1 compression, '50 had 7.0:1 for an increase of two hp.