Father in law's 300 EG 72571x bought new late 50's.
yeah, there was a pretty big batch of them around 725,xxx and then it stopped again. All of them were factory d&t after 750,xxx or so. Some earlier groups are out there, and I think the 638,xxx is one we've seen before.
This EG sold on GB a year or so ago, 53575x; Factory D&T with SAVAGE MODEL 99 on the left side; 250-3000 that showed pics of the receiver SN, matching forearm SN. 1949 with LBC A.
510,000 would probably be 1948 production and an EG to boot? Who knows? With Savage never say never.
Yeah, 510,000 is more than a year before the first 1950's 99R came out as far as we can tell. So it was likely a receiver that shipped at least a couple years late.
I have an EG #7277xx that is side stamped and D&T. We've talked about it before. I also saw an EG with a 75xxxx serial number that was top stamped and not D&T.