I blew it up and played with the contrast and tried to sharpen the image as much as possible. It looks like 813....clearly to these eyes it's an 8 and that would make more sense as a patent use number.

[Linked Image from i.imgur.com]


I went down the rabbit-hole pursuing patent use numbers hoping to find 1 maker who was reasonably consistent.
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=567037

The 1860 Henry patent 2802 - 7 groove shallow rifling seemed to have the most promise and for the first 5 years or so seemed to be chronological. However when A&T started providing patent use numbers it went haywire.
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=568234&page=1

I tried to get Purdey to tell me if they had records on 1863 J. Purdey patent no. 1104 - “double bite” under action bolt....Reilly regularly used that patent up to 1877. Purdey said their records are literally locked up and they cannot access them. So indeed they can't be used to date a gun....unless it is only to identify the last gun before or the first gun after the expiration of the patent in which case it can help with dating.

In my dating chart the last Reilly with a 1104 use number is:
20623 - E.M. Reilly & Co., New Oxford Street, London and Rue Scribe, Paris. 12 bore. Shotgun SxS. U-L, rebounding hammer gun. (Purdey patent 1104 use #3928)
My chart dates that gun to spring 1877 (getting pedantic - possibly late April)...which is certainly a sanity check on the validity of the Reilly Serial Number dating chart since the Purdey patent expired 04 May 1877.

And here are the questions that have never been adequately answered about Patent use numbers:

-- If the owner of a patent made a gun using that same patent, was a patent use number stamped on that gun? For instance are these Greener patent use numbers stamped on Greener made and marketed guns; or are they only used for others who purchase rights to purchase the components or build them under license?

-- If a gun maker built a gun under license using others' patents, assume he paid for the patent use number. And the corollary, how would one know if that gun maker actually made the component himself, or purchased it in the white from the original patent holder?

-- If a gun maker created a gun in the white using patents that were not his - for instance if Scott made a gun in the white for Holland & Holland, but used others' patents (Purdy 1104 for example), who paid for the patent use number and where was it recorded? Scott who built the gun or H&H who finished it?

In the case of this gun apparently it was finished by Reilly but built by Greener (in the white? - probably - because Reilly didn't serial number guns he didn't build; he did serial number guns he finished as did H&H etc.).

Owners of an actual Greener built by Greener and marketed by Greener at this time might note if there is a "G.N.PAT" with a use number stamped on their Greener factory guns. That would at least tell you whether Greener authorized Reilly to build it or sold the components intact.

(813 authorized over say 8 years is not an inconsequential number - 100 a year - so perhaps Greener did put patent-use-numbers on his own guns because Reilly and others were clearly not selling a lot of them).

But one suspects that every firm had its own practices and nothing was industry wide.