I'm a new member, but thought I'd share a little project I did on my H12 Color Case Hardened Carbine. I'm not saying if you decide to do it that you have to do it this way, but it worked out for me.
I really like the Big Boy, but one thing I didn't like was the thick recoil pad. I didn't see that Henry offered a thinner buttplate (BP), so I started looking around and that's what brought me here. I saw a couple threads asking about it, but didn't see any solutions here or elsewhere.
I found different ones on eBay. I really liked a S&W one (my initials

, but couldn't confirm any dimensions. I found a new Marlin BP, and the seller had factory drawings with dimensions and said Marlin Model 39 was one model it would fit. I have one, so I pulled it out of the safe and it looked like it would work.
Here it is compared to the Henry pad. It's about 5/8" thinner.
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