I've seen many of those old guns with Tennite stocks and even have owned a few.
It was an amazing accomplishment, for the era.
I've heard that time makes the Tennite more brittle,,,and that could be the case,,,,but I'm just not convinced of that, and here's why.
They were put on cheap guns only. Cheap guns in that era, especially, were subject to extremely rough use and often cared for badly. It's rare to find a "hardware store gun" from the thirties and forties that's survived down to us today with a walnut stock that hasn't seen it's wooden parts banged around and repaired, with chipped out stock toes and peeled varnish and "patina" (we used to call it rust) all over the metal parts, showing severe usage.
If the Tennite stocked gun was not subjected to extreme conditions, and it appears to be in nice condition, they will often still give good service today.
Stevens made cheap guns but they never made any junk.
It's just that when a Tennite stock broke,,,that the gunsmith down around the bend and up the hill couldn't repair it, is why they get such a bad rap today, IMO.
One of the worst things about the Tennite stocked single barreled Stevens was they were so light in the shotguns they would kick like rented mules and handle like a broomstick.
But that was also true of the wooden stocked ones, as well.
Enjoy the gun. If it breaks the stock it's not worth much less than it is before, and there are still wooden replacement stocks being made for them.