FTR means factory thorough refit. Basically reconditioned at a British arsenal.
If it's been in the family since 1950, that likely means that it was in the first wave of imports into the United States after World War II, and was purchased on the surplus market.
Or, it could have been brought back from the war as a trophy, althought that's not as likely given that it's an Allied gun as opposed to an Axis powers gun.
But, at some point in its like the stock was shortened, which was a pretty common "home gunsmithing" exercise.
A number of the markings "England" and "18.5 Tons" or whatever number that is, are consistent with British surplusing practices.
Nice close up photography, by the way. Most times we get closeups and they're unreadable.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.