I imported 4000 Sten magazines back in the early 2000's. All were near new, assorted Britishmanufacture and from Finland
where they obviously didn't see much use. I still have a couple hundred here if anyone is in need. I bought an Erb tube Sten Mk.V that was a total piece of crap. I sent it out to Paul Krogh in Colorado who cured all of it's problems with distinction. I refinished it when he returned it and still have and enjoy it. One of my few transferable guns.
I've never had a problem with Sterling magazines in my Mk.4/L2A3 and Mk.6 guns. Old and beat up or new in the wrap, they all seem to work perfectly every time.