At the time, there were many people listed as Engraver as their occupation.
Most were doing Stamp and Die work, a job that some Firearms Co in house engravers were tasked to do as well and took up a great amt of their time.
W/O that work being done (either in-house or by outside work engravers/die cutters), the production lines come to a halt.
All the stamps and roll dies were hand cut at that point.

All the industries used them, and recutting (sharpening) of the same was a steady business.
Every machine, fixture, jig, part that had calibration or other ID markings it, those markings were done with a stamp or die. Someone had to make those dies.

Print plates were also made by hand engravers. The older term of CopperPlate engravers comes from the hand engravers that did the reverse image engraving of scenes, portraits, of every 'picture' that was in print in a newspaper, book or simple poster.
These were done on Copper blocks for use in the presses,,the name copper plate engravers from that.

All that fancy border work on printed items such as bank note and govt bonds, mortgage and stock certificates, currency, Stamps, Military documents,,etc,etc.
All done by hand engraving a master reverse image plate.

Jewelery engravers were many as well. Some of the bigger shops in NYC and Chicago had several engravers employed in house at a time.
No shortage of hand engravers at the time.
..and don't believe the often told line about a Jewelry engraver won't engrave a piece of steel cause it's too difficult. Totally untrue especially when talking about the era of gun mfg in the 19thC and early 20thC.
Every one has their favorite mediums, but when these arms were made, the steel/iron used was soft.
If case hardened, it was no different than case hardened steel in todays guns. It needs to be annealed first,,cut then recase hardened.
(It can be cut thru with modern Carbide bits, but it looks awful as the edges of the cuts chip out from the brittle thin 'case'. Considered poor work)

Who could have engraved 'the other revolver'?

I don't know. As I said I don't know who engraved any of them.
I pointed out the differences in all 3.
Plus the few things that may be in common.

As I stated:
"...I'll add that it is very tough to assign a 'style' or cut to an engraver and be able to look at a piece of work and say 'so and so' did that..."

An example, at Colt F/A in the ten yrs from the middle of the 1870's to the mid 1880's, Helfricht was their 'Master Engraver'
But in that time period, Colt employed at least 20 different engravers, some of what would be considered Master skill level, down to apprentice engravers.

Helfricht didn't engrave everything Colt sent out the door. He was their main guy til the early 1920's.
But all those other engravers that worked there, all those that came and went, different skill levels, levels that likely improved over time with practice and mentoring from those around them,,,where is their work?
Some didn't make it in trade most likely. Other did.
Who decides what engraved guns are attributed to them?,,What where their names and how long and exactly when did they work there?

Same with the larger engraving houses that took in work from the factorys. Same questions.

Since most all factory and work done 'for a factory' is done to a pre-determined pattern with little or no deviation allowed by the engraver, how does one look at all the work and decide who cut what.

Does a person that is not an engraver understand what exact chisel points are used or can be used to make a certain cut, or one that can be subed for another? How about sharpening the graver, How is it done, what angles(s), how often, with what, what grit polish, hammer or hand graver, RH or LH'ded engraver,,can you tell?
Single point or liner, punch marks or graver,, or chisel cut.
Lots of things that are clues that an engraver sees go un-noticed by someone that does not actually engrave.
To ID those points usually still not be enough to use to then ID a specific engraver that did the work.

The running thought is that there were about 200+ active firearms engravers in the USA betw 1850 and 1900. Most of German and Austrian descent.
I don't know where that # comes from. Someones guess or maybe a big list of engravers exists. If someone has that list of names and where they worked, I'd like to see it!
But for me, it does not sound unreasonable. Not all working at the same time of course, at least I don't take it that way.

I would adjust the statement to read just 'engravers' and not 'firearms engravers' however.

There were plenty of candidates for a possible engraver for any of the 3 firearms shown.
Who engraved anyone of them is unkn to me. I would not place a guess based on any sort of analysis of the engraving cuts, style or layout.
They are each different and there were a lot of active engravers at the time.