Uncle's Retirement Watch

As I've been registered on here for two years but with no posts, I've got to surmise all prior presence here has just been for searching. I've just spent 7 hours today refreshing myself on the sketchy notes I put together then, along with relearning the 95% I've forgotten. I've hit a big roadblock after having been able to find out a good deal about my uncle's watch using what appear to be popular online sites for researching. However, I'm striking out with where to go when I can't find anything resembling an actual serial number!

It appears an image hosting platform is necessary to post pics, so until then I have found enough to be pretty certain it's a Seamaster 1020 166.0209
Gold Plated Bezel, Stainless Back
White dial, with 'Omega' and 'Automatic' under that (the only markings)

Background: He worked for a huge multi-national corporation, and suffered a major stroke in '79 so they retired him out. That, plus his personal accountings before death go toward legitimacy of the watch being genuine goods.

The back cover reads his name, corp. logo, and "For 25 Years Service - 1965", which matches retirement date. I surmise the protocol was awarding quarter-century anniversaries and they reverted back to the award level prior to the 50-year mark.

Identifications-
Dial side of back: Above the triangle is '5491', and a hand-scratched or penciled 'K91ACFR' upside down and running outside the rt. hand triangle leg

and the 166.0209 at the bottom below all the Swiss printing

Movement:
'OMEGA' and 'SWISS'
'Seventeen 17 Jewel'
'Unadjusted'

(Inside of the indented circle)
'39'
'407 830'

and the '1020' near that circle

There's nothing else, certainly nothing close to resembling the MAPICS formatting of that timeframe:

• First System (before 1962): XY 1234
• MAPICS System (1962 – 2007): XY.123.456 & XY.123.4567 (from 1970)
• PIC System (1988 – 2007): XXXX.YY.ZZ
• PIC14 System (from 2007): AAA.BB.CC.DD.EE.FFF

I've read 'Every' genuine Omega got a serial no., no exception.
Is there potential the number is hiding under something else, requiring a jeweler visit?
The entire flat field of the back is filled with the inscription. It has a brushed finish, different than the sloped polished edge. Did some ham-handed engraver retailer flush everything and remove the serial no.?

And a few questions on the id I found nowhere online:
What's the 5491 and the handwritten script inside the back ?
What do the numbers signify inside the little depressed circle on the movement?

I realize the modest valuation this vintage and model holds in its very best of condition. Without all appropriate documentation to start with, most importantly the serial no., I have trouble validating the cost for the company's complete restoration services. Last I read that mandated going through a watch retailer and ran anywhere from $400-800.

I've long crossed the line into TMI. Any help to either encourage spending some money or just 'get over it' is appreciated. And funny, the two prior image hosting places I had used decades back went each in turn to paid subscription. Time to hunt down another one. Preferably free.

Gad Zooks! Even with my meandering with the details you folks beat that to the ground quickly, thanks! That Omega illustration was initially what started me down the path to the rest of what I pieced together. I admit a big D'oh! with the Ser. No., but hadn't considered the leading letters could be numerals. And curious no graphic ever popped up in all my twisting searches that displays it in that format.

And appreciate the primer on attaching pics. I had used the Image button earlier but caught the prompt for a URL which scared me off. Following through with the procedure this time led to success:

I'm relieved enough that my nosiness has diminished about what the only remaining unknown marks designate:
"Dial side of back: Above the triangle is '5491', and a hand-scratched or penciled 'K91ACFR' upside down and running outside the rt. hand triangle leg"
Back is indeed s.s., just had wife shine diff. light attempting better detail:

My uncle and I had been close, and he lived quite impaired for 20 more years after his stroke. It's evident the watch is exceptionally worn and needs full service, and I'll be moving ahead to research the description and cost of Omega's refurb services beyond the little I found out years ago. If not for me then for my Millennial son if he considers a value in it. I'll keep y'alls updated.
Thanks again.