Another update - finally got around to partially pulling apart my Merkel.

There was a small bit of timber on the sear for the LH barrel but it wasn't actually on the sear face, and this lock work isn't the one that doubles anyway. Have a look at the photos - the LH trigger assembly has a recoil operated sear block, whereas the RH doesn't. Maybe this actually stops it doubling on RH barrel first? I could pull it out and see if it does, but I won't (ain't broke, don't touch).

That's as far as I got. I will pull out the hammer at some stage. The sear face on the RH barrel looks okay and I'll have to pull the hammer out on that side to see if there is anything on the matching surface face compromising full engagement.

Also noted that the recoil lug is at the very rear of the tang and looks to be bedded - there is a filler between the lug and the timber.