My friend, you hit the jackpot! Assuming you paid the $500 mentioned, you got it for at least 1/3 or less the going rate, probably less. Yous was one that was sent to the UK in 1942. The Britishproof marks identify it as one that was imported back to the US, probably in the 1960s. The markings look a little worn, but you have a (based on previous sales of these rifles) a $1500-2000 rifle!
If you want to find out more about these rifles, read the article that John Beardand I wrote in the July/August issue of "Man at Arms" Magazine.
I have #3,024,801 and the markings are very similar. However, it was obtained earlier, so the proof markings on te FRSB are missing.