So I’m currently reading through Halo: Last Light (I’m really late to the party) and throughout the book it references Marines using MA37 ICWSs as of 2553, I was under the impression that the MA37 was exclusively used by the UNSC Army is this an oversight by the writer or is there a canon reason for the spontaneous shift?
More than likely its simple restructuring; 30 years of war will grate hard on whatever supplies you have.
Since the weapon is functionally identical to basically every other MA series weapon out there, if that’s what the UNSC had available in 2553 it would be what they hand out.