That gun appears to be a 444S produced between 1972 (when the model changed to a 22" barrel and the straight grip was eliminated) and 1982, the final year Marlins were made without the CBS. This gun does have a grip cap, just hard to see.
Nice catch. I hadn't noticed that. It's clear that it's the wrong stock, when looking at the full size image.
Looks to me like a replacement stock (probably from a 336W).
And the wood seems too plain for a 444. (444s generally got pretty nice wood for a factory rifle - with more figure and chatoyance than most 336s.)
There's no bullseye, either.