I have a Berreta O/U 12 ga.Its the 686 Essential,26 in bbls.It made light.There is air between the barrels.I think I was using 1 1/8 oz AA Pigeon loads for a dove hunt.I was working nights,worked Fri night,drove a long way,had no sleep,and for whatever other reasons,I was placing the butt on about the same place and my arm(missing all the birds) turned purple,too.
With those light loads,my shotgun is not a brutal kicker,I'm suggesting that part of the arm is not good for recoil.
While that curved butt may not be the best,at least its not a Crescent like a Hawken.
Right now you have a very nice rifle that may not be a good fit for you.To fit a modern rubber recoil pad..to say it nicely,would compromise the rifle's value.
Maybe Shilo would have an option of making you a new buttstock on a more Creedmore pattern with an original type shotgun butt.
I'd encourage a more upright shooting position,and tune your position to place the butt on your shoulder.Getting off bench is a good plan,and sitting with crossed sticks may be a good position.
I recently discovered the P.A.S.T recoil shield as others have suggested.The work very well.The 1/2 in thick is what I prefer.
Going to Fg may help a little.
I would expect you will have some flinch to get over.Too much recoil develops bad habits.
Keep it fun!