OK, here we go.....
Those stainless steel jobs were almost certainly adjustable butt plates like the Jones or the venerable Morgan. Or, one of the recoil management systems like the Graco. All these add weight at the back.
I recently extended the stock on Number Six to fit my gorilla frame. I had a smith add an inch's worth of black fiber spacers and a Decellerator. The spacers are nigh weightless and the pad weighs just enough to balance
things.
The LOP is now 15" which works well for me when shooting low gun. For premounted games like trap, 15 1/2" works better.
Note, since the spacers are formed to the lines of the stock,the pad is now a bit larger than before. This spreads the kick out a bit and aids comfort.
The WOP gun here(My Beretta White Onyx Playtoy) has a Limbsaver pad that brings LOP to 15". It's a hair thicker than the Geltec it came with.
In your shoes I'd mount a slipon pad OVER the regular one and play with the length by adding shims until it feels right and prints where you want it to on the patterning board.
KickEez's Magnum pad is a real kick stopper. It's 1.4" thick, which can help add some length.
HTH, sing out if there's anything unclear.
ADK, we're talking about extending stock, not mags here, but try Choate. I believe they have a 20 gauge extension for the 870 which should work....l