Clubmaking Forum

Assuming you are talking about the club MPF charts...  The 6 means that they measured the 6 iron for that club.

Ignore the highlights, those are my numbers but this tells you how to read them.

https://www.golfworks.com/head-mpf-ratings/ Titleist is only through 2021 but Callaway, Mizuno, Taylor Made etc. all have the 2023 models.

I don't know about any studies, but there is an episode of My Golf Spy's No Putts Given out there on YouTube where Paul Wood is talking about how hard it was to get players out of the S55 irons.  He mentions...

Looks like the RAC wedges are .355.  This is a handy reference tool: https://www.golfworks.com/oem-golf-shaft-replacement-guides/ They haven't been updated in a couple of years, but still helpful for...

They under "The Golfworks" when searching by manufacturer https://www.usga.org/ConformingGolfClub/conforming_golf_club_results.asp