Tamron Adaptall 300mm f5.6

The latest car boot find, bought for a pound, which I thought fair, if only to get the adapter!. Retrospectively referred to as adaptall 1 to make it easier to distinguish between the lens series, the most obvious difference is the grip texture. Attached to the Pentax KM, as they are roughly contemporary, and it had a K-mount adapter on it!
Oddly, the lens doesn't focus anywhere near the infinity mark on the scale, but some quick snaps on the E-500 seem to be achieving infinity focus! Possibly someone has had it apart and misassembled it, but I would have thought that would have prevented it from focusing to infinity altogether. Although I have come across some lenses where the distance scale is rather "approximate" (mainly zooms), I wouldn't have thought a Tamron prime would have escaped from the factory like that.....