[updated 9/19]
I wanted to bring this one up from the archives, because concerned-netizen
Kwikdraw has resolved the mystery. That storefront is the premier custom airgun shop in Japan, and possibly the world. Rock!
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June 08
this is in Shibuya. I tried to go down to the shop. a spiral staircase led down to a locked metal door with no signage and no handle. perhaps that's just as well.