Pentax 50-135 - why is it always

So I took the replacement out to a lounge where a friend of mine was playing guitar the other night and it locked beautifully in really low light.

Today I was out taking a few autumn shots, put the lens on, and it wouldn't fire. Just like the 16-50 would do sometimes, so I took it off and put it back on and it worked.

There are others in the lens database on the other forum with higher serial numbers entered in 2010 so this is obviously old stock. I think I should get one from new stock when they come in.