A very nice Lemania indeed!
Congrats on a great pickup!
馃憤
I have collected serial numbers with confirmed purchase or dedication dates.
Yours I would date to 1954-1955.
The dial design is very interesting. I have never seen a black version with that pattern before.
What I do have is a cal 4651 Automatic with a similar pattern in silver.
The ref 280KO appears also in my 1956 catalogue.
It came in A-Acier and KO, Gold on Steel.
Note that the reference number is a case reference only. Lemania had a modular and small scale production. You may find a great variety of sweep seconds and sub seconds movements (3000 family) and dial variants for the same case ref.
Prices in GBP 1956.
And my 4651 Automatic.
Cheers.