Dave, I can’t comment on 7.62x51mm but below are 9mmP headstamps manufactured in or for the two countries.
The round on the far left is Burmese. The source was the Hong Kong Crime Lab before it was transferred to the Chinese. The symbol at the top probably indicates the Burmese Military Ammunition Factory. The two symbols on the lower left identify the date as “72” and the symbol at the lower right is “9” for the caliber. The Burmese date can be converted to a western date by adding 638 or 639 depending on the month, but the “72” appears to be a western date.
The middle round came from Aberdeen many years ago. The symbols across the top are “Burma Ka”. It isn’t clear what “Ka” character represents. the characters at lower right are the numerals “56” probably representing the date from the western calendar since this round was obtained long before the 1994. The symbol at the lower right is unidentified. If the date is actually 1956, then it seems unlikely that this headstamp would appear on 7.62 NATO cartridges. In his book on Military SAA from 1945-1980, Peter Labbet identifies the headstamp you illustrate as used on 9mm Para ammunition (Pg 52).
The round on the far right was obtained by the friend of an acquaintance who was stationed in Lebanon with a UN Peace Keeping unit and bases near or with a unit from Nepal. I was told he was given both a 7.62x51 round and this 9mm round. I have no idea what headstamp was on the 7.62 round.
Bottom line, Headstamps from both Nepal and Burma exist in 9mm so almost certainly exist in 7.62 since both countries used 7.62 NATO weapons and as a general rule, small countries produce their own rifle ammunition before they produce pistol ammunition. Having said that, the existence of the headstamps does not guarantee the cartridges were produced in these two countries. There is evidence that Myanmar/Burma produced their own weapons and ammunition for many years going back to the “Ne Win Sten” (BA 52 machine pistol), developed and produced with support of the Fritz-Werner of Germany beginning about 1953. Nepal could have purchased their ammunition off-shore but they could also have purchased the equipment to make the cartridge shown above which has a high quality headstamp.
In a separate email I am sending you some other assorted info.
PS: The Burmese translations are from a native Burmese speaker familiar with weapons.