If the amount of weapons the Irish Defence Force have supposedly released in the past 30 years is indicative of anything, it is that they had the largest Land Forces in Europe apart from the old USSR.
The '40 inspection mark is just that. It went through the factory again in 1940. The principal reason for this is to replace worn out gas cylinders. Gas cylinders and replacement locking shoulders to correct CHS were a return to factory repair until early 1944. Much more to it than that of course but.......
On the other hand, it is possible that a gun could have a production line fault that was discovered late in its manufacturing life. This was not uncommon and such guns were diverted to a rectification bay or what was known as '....the hospital' where guns could be rectified and sent out for service instead of scrapping them. Some that went through the bay had an 'A' suffix added to the serial number. So it could be that the gun was eventually rectified and then proofed and out-examined yet again afterwards. By this time it was 1940 - when the early C to late F series numbers were produced.
There are a few anomalies like this known to exist.