They sold aftermarket ones made by AWT.
as I said in my post above , getting it to the power could be a hit or miss.
So a adjustable one via a valve may be easier to fine tune.
If you fitted a AWT one to a rifle and it was too high power wise it's possibly going to take a fair time to fine tune. And as I suspect , the rod in the ram being hardened may have been seen as too much of a time consuming job for mass production.
If say it took 3 hours or so for the home tuner to fit one , it may have taken the makers the same time but time is money to them .