I've got an early MechTech CCU with the strap metal welded stock.  The new units accept adapters that can take a fixed or adjustable AR style stock/tube, or a wire collapsing slider.  Mine has a steel disk welded into the end of the housing tube, and the strap metal welded to that disk.

The strap stock is not comfortable; the 'adaptations' Mech Tech used to provide are either no longer available or quite expensive, and my home-made attempts to put a comfortable comb and flat recoil pad on it haven't worked out.

MechTech makes a replacement housing ($60) to allow an old unit to use the new adapters, which cost at least another $35 (plus S&H).  But in my bag of stuff I have a threaded aluminum adapter designed to accept an AR buffer tube; its very similar to this one but doesn't have that 'button' projecting out of the flat back; just the two slots with bolts.  Ut must have been in a sack of parts I picked up some years ago; I'm not sure what it was for.

If anyone out there is familiar with the older CCUs, I'm wondering if its feasible to do the following; I don't have the option to weld right now:

Cut off the strap stock and grind the rear plate flat
Drill the CCU end plate for bolts going through the adapter slots.  Might tap the end plate but its not thick enough to do the job alone
Might drill a smaller center hole in the adapter and end plate (and thread the end plate) for a 'locater' screw just to keep the adapter from potentially sliding up and down (slots...)
Use nuts with loctite inside the CCU, bolt on the adapter with two bolts (and optional third sans nut), and divot out the buffer to fit over the nuts (and glue it back in).

Then screw in a buffer tube and attach an M4 buttstock

Does that seem like it would be strong enough?

You can in fact use the pig nose at adapter from Ace rifle stocks. That's pretty much what the newer conversions are like.

Thanks, going to give it a try.  If I use 1/4" bolts I will have to slightly open up a spot on each slot, which will obviate the need for a third locater screw by preventing the adapter from moving up and down.  I picked up the hardware today.

You can also buy the one without the boss on their site if it makes it easier. I'd buy the one with the boss and file it down to make your "locating" pin.

My pig nose didn't have the protrusion, unlike the one listed on Midway.  I'm not sure I'd like to try to drill and file a compatible hole in the CCU endplate anyway.  Better to get the two main holes drilled and bolts installed, then drill a small one in the center and use a roll pin, or drill and tap and use a smaller bolt.

Or just do 3 corners of the slots or hell all 4 corners!

Believe it or not the finickiest part of this is hogging out the rear off the big rubber buffer to accommodate the nuts.  I'm trying to minimize how much has to be done.

I've got the pig nose bolted in with two bolts, using thin nylock nuts and loctite.  I'm going to drill a tiny hole dead center and drive a roll pin in so it acts as a position lock.  It will be short enough to not stick out into the buffer.  Even with just the two bolts, with buttstock attached, there's no sign of movement.  Its going to be much more pleasant to shoot, and once I build up the comb a bit (no charging handle to worry about) it'll be perfect with the red-dot.  So far I'm very happy, and I didn't need nearly $100 in new parts to do the job.