It's a post-WW2 (late 1940s) knife SIMILAR, but NOT identical to the USN MK1. The metal thing on the handle is a post purchase addition.
A WW2 USN MK1 would have USN stamped into the ricasso opposite of the KABAR stamp.
These are really nice knives. They were made with 2 blade lengths - 4" and 5". The ones sold as "Official BSA Scout Knives" had an etch on the blade, which usually wore off with use. The handle diameter on these feel slightly smaller than the MK1s and really feels good in my hands, probably because I have short fingers. But I never seem to remember to put calipers to them. Note to self - dig out the calilpers.
They might have also been sold OTC without the etch as well, but if they were, they must not have been advertised very much.
They are similar to MK1s in that they have the flat pommel construction or the Kabar MK1s. The MK1s were 5" blades only.
Some have attributed the model number to the as 598s, i.e., 598-4 and 598-5, but no one has ever seen any advertising specifically stating what Kabar's model number was for these.
Gunsil, our resident "pre-1960s Kabar expert", has been on the prowl for definitive model numbers since dirt was clean, but the info has been elusive.