You are right that there are Marine rifles that sometimes will show up with these markings. I actually have a documented Marine 1903 with a stock marking like one of the rifle's pictured above. But just knowing the rifles history, I just highly doubt it was applied by the Marines.

It's a long story and I don't want to take over the Op's thread. But the cliff note version is I have thousands of documents now from the archives that detail the Marine 1903's. And even though I can detail about every known Marine trait and even some traits that aren't known yet, I just can't find any mention of them doing this to their stocks. In fact every mention of Marine stocks just leads me to beleive this wouldn't have been done.

But we are usually pulling hundreds of Marine Documents every week from different archive locations, so maybe I will prove myself wrong someday and I will find it. I will be the first to admit I've proved myself wrong many times before.