I know someone here has an original.
I'm looking for ID, OD and OAL for the upper barrel jacket and lower mag extension sleeve for this:

I have an extra 870 barrel I'm going to shorten to 21" for a clone.
Any input appreciated.

For starters, it's a "one off".
I don't mind spending hours and hours doing a DIY project, if it is feasible, and repeatable, see where it goes from there.

I have the set up as pictured but it is missing one of the screws. Does anyone know of a source for them?

Just noticed this thread as I don't cruise by here very often.  I can supply dimensions if you still need them.  I recently purchased a really beat up original that looks like it was carried by every Marine raiding party from the Mayaguez incident until last week.  It's currently in pieces on my bench so it's a good time for measurements before it gets refinished.  I might also have a correct Remington rifle sight rear base but will have to look around.  If you don't have access to an original 21" barrel, I'll get measurements for the proper location to braze the sight on.

I can also supply replacement screws from a small batch of reproductions made a few years ago.  They are unique to the USMC version as the USAF bead sight adaptor used a slightly shorter screw.  They are $10 each or $25 for a set of 3, delivered.

Rick

@ordnance

Can you post pictures of yours?

For those interested, I just posted an ad in the Shotgun Parts Forum of the EE for these screws.  And I'll make a correction to the previous post...the screws ARE correct for the Air Force type muzzle adapters found on the Ohio National Guard guns.  I just dug out my AF adapter and measured them to be sure.  However, they may be too long for some of the commercial/police type adapters Remington sold.

As requested, here are a few photos of a USMC bayonet adapter.  It's a work in progress, freshly sand blasted after a lot of filing and deburring and fitted with reproduction screws and the proper Remington front sight.

Very cool thank you

Bravo! That is some excellent metal origami.

IM sent. Much appreciated.