Well, the VIN number you gave M41870 is in the correct sequence for a 1957 Utica Bend. The contract number is also correct for a 1957 Utica-Bend
The number Medlog has supplied is for REGISTRATION number 4M41870 - which is for a 1970 Kaiser-Jeep M35A2, with the VIN of 0525-16631. This truck was built on contract number DAAE06-70-C-0001. With all the thousands of numbers running around on these trucks, this would be a very easy mistake to make.
The VIN number is stamped on the frame rail - if its a 1970 K-J the 0525-16631 number is near the driver's side shock mount. If it is a 1957 Utica-Bend the M41870 is PROBABLY on the passenger's side, between the centerline of the front axle and the rear of the fender.
The registration number should be found in one of the following locations - on the doors, on the inside of the truck, on the underside of the hood. Failing that, wet the hood and look near the juncture of the hood and the engine compartment side panels. You should be able to see/feel the numbers. Likewise with somewhere on the tailgate.
At this point, I am 99% certain it is a 1957 Utica-Bend. As you may have surmised, I am home now - and can tell you for a fact that VIN number M41848 was a 1957 Utica_Bend - and while it is possible that by the time the next 22 trucks were built (yours) the year had rolled over to 1958 and the maker to Curtiss-Wright - its just not likely.
Hope this helps,
David