Dad was a Captain of the security force at the Idaho National Laboratory, started right after he got back from Korea, late 52-early 53. Back then, they had model 10's, they retired those, and gave 'em all 4" Model 66's. If any of the earlier guns were marked, they'd have been "AEC", or "USAEC" for the US Atomic Energy Commission, and the late guns, if marked, would've been Department of Energy, or "DOE". After the M66's, the federal civil service security force was phased out, and contractors took over the security.