Probably...
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Theres another one listed on the 1911 forum that has the same makings, but on the frame this time:
Colt 1911 govt serial C61535
I wonder if the the proof house applied the markings under instruction?
In my opinion, the Proof Houses, being British, would know not to use "British Government" which I think that we all agree would simply not be used this side of the pond.
BTW, I just noticed: the first picture is Birmingham Proofed and the second London but the "Released etc." marking is identical. It's not like the two Proof Houses to use the same mark.
I still reckon that this mark is to un-British to have been applied here.
I'd guess they were applied by the British exporter. There is no US requirement for those type marks on a post 1968 import. Importers here have their own set of marking requirements. There have only been a couple of windows in which importers in the USA post 1968, could even bring in Lend Lease curio and relic firearms legally. This was only once you obtained a retransfer authorization from Dept. of State first. Without it, BATFE Imports Branch wouldn't approve the import permit. One window was under the Reagan administration in 1986 which ended in 1988 I think under GHW Bush with his import ban on military semi auto lookalikes and the other during the GW Bush administration in 2005. The Obummer administration ended it again in September 2013.