I don't know how many were stamped like yours, the 7.62mm was later moved too the chamber. There is alot that is unknown about the 7.62 Navy rifles like extacly how many were converted with the insert and later pulled and rebarreled with the 7.62 tube.
What is known is that H&R did the original insert and also rebarreled with 7.62mm tubes, AMF also rebarreled with 7.62mm tubes each contractor stamped there work. H&R did not stamp there insert conversion as they were the only contractor. The Navy did rebarrel there rifles and there not stamped, I have a new Navy barrel that I installed on my rifle so there is no contractor stamp just like the Navy did the conversion. I would not sell someone a faked rifle but there are people that will so If you encounter a rifle make sure there is USN/USMC release papers that go with it. There are Navy 7.62 barrels floating arround there few but out there.
The Navy started with the insert and installed them in any barrel that was on the rifle, SA#11010457 SPL is the frist run of 7.62 Garand barrels and the second run was SA#1168514 both are 7.62mm tubes that the Navy used, I have seen production figures of 3000 for the SA#1168514 and that they were all air gauged the best tubes(10%) were pulled for the Navy Rifle Team, I have not seen any production figures for the SA#11010457 SPL barrel so its hard too say how many Navy 7.62mm M1barrel there are total.
I will further add some confusion too the Navy 7.62 barrels, I have seen a few Air Force Permium Grade(AFPG) M1 rifles with Navy barrels, AFPG was/is stamped near the gas cylinder.
SA closed in Apr of 1968 and shipped there machinery too Rock Island, well the AF must have like those Navy barrels, RIA #11686320 SPL M118 6/69 was made, I have seen pics of one AFPG M1 from the CMPAuction with this barrel, so how many did RIA make for the AF?