The Tanaka works AICS's were made out of cast zinc which was "blued" - sometimes the Zinc oxidizes leaving crusty white residue on the outside. see if there's any of that on the magazine's body and the inside of the Magwell.
I've had a look and I can't see any white residue. I did take a picture (sorry for the poor phone focus)
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I've circled a few areas in red in the mag well where there has been some rubbing between the mag and the mag well wall and the mag release catch to the point where the bear metal can be seen. The long style mag can be inserted about 1-2 cm before it starts to require more force to push in, and once it feels like it's fully in, it still has a bit play and doesn't engage the mag release, I can just pull it out (with a bit of force).
I've read somewhere that if left with propane or HPA in the mag, it is possible to warp the mag, could this have happened as I did leave some green gas in it over the years to keep the seals good, although it wasn't full.