You, know what, Alan? A magnet will tell you instantly if it is steel or some sort aluminum alloy. Steel or brass will be heavy. Steel magnetic, brass not magnetic. Alloy will be light, and not magnetic. Aluminum alloys can range from something almost as soft as lead, to something they use for armor plating in hardness. The only problem aluminum on aluminum is not a good bearing surface (tends to gall and stick, think of filter rings. and this is the answer to your question below) so is not usually used by good engineers. Aluminum on brass works very well.
Funny you mentioned this. The magnet idea came to my mind last night, but it was too late to experiment. Now I have to figure out where to get a magnet for the job (don't want to strip my radio). Unlike the HOYA filter threads which are quite soft (except PL/CPL), the retaining rings of Pentax lenses are hard and won't be stuck. But it could be aluminium alloy as many have already pointed out. I'll let you guy knows when I finished the experiment. :-)
regards, Alan Chan
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