There are a couple of good reasons why one might choose a WWII Camillus Mark II over the KaBar, PAL, and Robeson Shuredge Mark IIs.
First, Camillus and KaBar made the vast majority of WWII Mark II knives. This makes them much easier to find and less expensive in today's market -- often a LOT less expensive. And by the way, the ONLY difference between USN-marked and USMC-marked Mark II knives by the same company is the marking.
Second, Camillus made the least number of changes to their WWII design than did the other three manufacturers. If you compare a WWII-made Camillus Mark II (marked either USN or USMC) with a 1970's-made Camillus Mark II (marked U.S.), they are quite similar. This is because their initial design was quite robust. And, in case it matters, the US-marked Camillus Mark IIs are just as good and even less expensive than their WWII elder brothers, but their leather handles are a bit "rougher".
BTW, all Mark II production ceased after V-J Day. It didn't resume until the early 1970s, and Camillus was the only one of the WWII makers who resumed making Mark II's under military contract. All -- repeat ALL -- of the KaBar Mark II knives made after 1945 are "Commemoratives" intended for the commercial market. They're good knives; they're just not MIL-SPEC.
Another key price factor about WWII Mark II knives. The early ones had their markings on the blade - specifically on the ricasso. However, in late 1943 the War Department specified that they should be moved to the guard. There are a LOT more of the later guard-marked versions, which makes them generally less expensive.
Hope this helps.
Regards,
-- Nighteyes