Kev, I think you've hit the head on the nail. I take these old pieces out of the box and they seem to have a weight to them that the new one's don't have. Better rubber with more ingredients, maybe? And the finish is very fine compared to the new stuff. I have a lightly used mint Selmer BA alto that has the original Selmer HR Table B mouthpiece in the case. This mpc is just beautiful. The workmanship is like nothing you'll see today.
I paid $425 for my first MK6 tenor in 1973. The horn was used and it was from the 153,xxx series. I sold it for $4000 ten or so years ago when I bought the tenor I have now, which cost $4000. So I'm playing a near mint 103,xxx mk6 for $425 cash outlay.
...might be due to the elemental lead that was once routinely part of the mixture. Old hard rubber/"Ebonite" had lead added, I believe for better machining of the facings, bores and tone holes.
Rubber, in the forms that we use it, is a mixture of various things rather than a chemical compound of them. The rubber itself, a natural plastic, is a chemical with a complex formula, but all of the other stuff added (carbon black for color, sulphur for heat and cold stability, talc for God knows what, and lead as described above) is mechanically mixed with the rubber to give it its final qualities.
The raw rubber (which is runny, gummy substance) and its additives (which these days usually includes some synthetic rubber) are "milled" in a mill consisting of two diablo shaped rollers. When first added, the rubber and the additives can be discerned clearly, but as the rubber "mixture" is sent through the rollers again and again, it gradually mixes everything up to what appears to be a homogeneous product. However, it is all just layered together in infinitely small layers, and (with the proper equipment and heat) it can all be separated again, as bizarre as that seems.
Why do I know anything about rubber? Well, aside from being nosey as hell, I also used to regulate a variety of rubber factories, including tire plants, gasket plants, devulcanizing plants (those took the rubber (old tires, mostly) apart and recovered the raw rubber stock in the process), and others. Fifteen or so years of that and a little of it rubs off.
Rubber facilities are notorious for milling up employees with the rubber stock, as the mills are all manually tended, with the operator manipulating the ball of the stuff as it is worked through the mill. Get too close and you ended up going in with the rubber - not a pleasant experience.
However, the absolute worst was the devulcanizing plant, located south of East Saint Louis IL. A more vile smelling and uncomfortable place to work you could not imagine. However, they fed old tires in one end of the place, and got rubber stock out the other, so it must have been worth the fires and explosions (and our attention) to them.
One more thing about rubber. Although synthetic rubber (the various -prenes that Monsanto seems to produce a lot of) comes out of a continuous process (which has to be interrupted to remove the product), all of the other uses of the stuff are batch fed - i.e., there are no continuous processes involving rubber. This is relatively rare in modern industry - one of the few remaining instances of artisan production of a modern raw material.
I did a surface wipe on a very old Selmer mouthpiece (signature and facing stamped on the facing) with one of our sampling pads and sent it in for analysis. It only came up in the very low micrograms, enough to be a factor if you were grinding or turning the stuff, but not enough to worry about putting it in your mouth. (There are some "authorities" who maintain that there is no safe threshold of lead exposure, but they are considered to be alarmists.)
Just don't drill holes in it or grind your teeth on the bill or biteplate and consume the shavings...
Terry, thank you for that nicely detailed insight into the manufacturing process that goes into the making of a HR mouthpiece. I see that you mentioned ebonite. I have an old HR Woodwind NY bari mpc that is marked "Steel Ebonite." Do you know if the "Steel' they refer to would be the lead content, or maybe something else?