Idea is: most/all of the 'big' countries take a long hard look on the StG-44, decide that it is a worthwhile gun, and decide to introduce a version or two of it in their armed forces from late 1940s on. Whether with original ammo, or ammo that can fit, and you can 'invent' it (5.56-7.92mm x??, what ever floats your boat, while it can easily fit on the StG-44 base). The off-springs being viable wepons by 1970s. Note that this decision does not butterfly-off the 'full power' rifle cartridges used on machin guns used by infantry in the Cold war, but might play a role in cancelling some post-war SMGs and semi-automatic rifles.
What are benefits, what are shortcomings vs. what infatry usually carried between 1950-1970?
The FN FAL was originally in 7.92x33 caliber. The Brits had their .280. The Spanish their 7.92x40 CETME, while of course the Soviets had the 7.62x39. The French experimented with a 7.65x35 MAS using Mauser engineers (who would later go on to develop CETME and the Spanish round/rifle, plus the G3 rifle).
The US was the outlier and the one with the most power to ensure all the others conformed to the full powered battle rifle round.
The best bet is have the British .280 get accepted (the Belgians also developed the FAL in that caliber); though not a true intermediate even in the first low powered version, instead a very low powered battle rifle round, it was also supposed to be a new universal caliber for the Brits. I suppose you could get interesting and have NATO like and adopt the 7.92x40 CETME cartridge and rifle. Supposedly it did pretty well during an unofficial test at the Aberdeen proving grounds in 1954 and got high praise from US officials there (pp. 152-159 "Full Circle: a Treatise on Roller Locking"), but the decided to pursue what would become the M14.
If adopted it would have been a formidable foe to the AK-47 and do everything the M14 and FAL could not.
That is why I said that the Stg-44 is and was inferior to the Rifle No9. Why would other nations ITTL accept an inferior weapon to one designed one designed and built to their requirements.
Not really inferior if it already did what the EM-2 did 6-7 years earlier with all the kinks worked out.
The StG45 probably would have been the model adopted, as it was the continued development that the Germans themselves were replacing the StG44 with.
https://en.wikipedia.org/wiki/StG_45(M)
Cheaper, easier to make, not much heavier and possible to improve vs. a bullpup technology that was still finicky and a recoil too heavy for the system to meet the requirements the British set up for it (thanks to trying to get the .280 to beat out new the US 7.62 full powered round).
Have them adopt it in say 1946-47 after having their engineers improve upon it a bit (say get it down to 7.62 to use existing barrel boring machinery) and you've got something the US will have a hard time trying to supplant later, especially if the Brits adopt the 7.62x51 NATO as their 'machine gun' round.