I have an XS scout mount for my Marlin 336 - bought new about 18 months ago - and it's not a drop in fit. The hole which fits over the post you slot in the dovetail doesn't line up. It would need a fair bit of opening up.

Anyone else had/ heard of similar problems?

I would think they would have to be adjusted most of the time. My set for a Marlin 336 in 35 rem came with directions that said you would need to adjust the slot size. The directions said use a knife. That would be difficult. I used a dremel tool with a small grinding stone attached. I assume the dovetail distance from the receiver tends to vary.
Good luck. i love my XS set up.

Good to know it's not me then.
Cheers mate!