Way back machine.

I saw a complete Master Key mount on GunBroker about a month ago. The mount itself is unregulated, so anyone can bolt it to an 870 and register the 870 as a SBS.

Have not seen any 80% M203 receivers offered.
I think most people are happy with the complete Title I receivers (when they can get them), or buying the whole unit as a Title II DD.

Both of these have pretty small markets. The Master Key market is smaller.
They're cumbersome/awkward anyway due to their weight. It's better to have a separate 12ga. weapon/breacher (as all of the professional users found out pretty quickly).

C-More offers the M26 to civilians, but it's about $3,500.

Another option:

https://www.lockharttactical.com/category/product/lockhart-tactical-ubs-12-under-barrel-shotgun-mount-fits-870-clones

If only it were legal with a Super Shorty (guessing it would change it from an AOW to SBS).

There's a 2011 ATF letter to the contrary:

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https://www.indianagunowners.com/threads/aow-as-a-masterkey-not-illegal-per-batfe-1-19-2011.131233/

Yep, not a sbs but stays an aow.  I don't like the Lockhart version because the shotgun sits so low away from the bore axis. The masterkey mount seats the shotgun very close the rifle barrel, a la m203, which helps with balance and sighting and less bulkiness. I'd gladly pay a chunk of change for a masterkey mount clone.