Not sure on the different Sten as the pictures may not have been in the same time frame as he is wearing totally different kit in the smaller pic plus in the heat of battel weapons can get shot out of your hands so he procured another!
Some trivia;
I worked with a WWII vet when I was farming he became a POW of the Germans and said his 303 rifle was shot out of his hands just prior to his capture, of his confinement he said little except being one night caught outside the wire stealing potatoes.
The guard said don't ever let me catch you outside again or I'll have to shoot you, and with that John went back with stuffed pockets for his hut never again did he go outside........