C1950 gold Seamaster small seconds - unusual serial number

No, not a serial number.
A 1950 Seamaster would have had a serial number something like 12,123,456.

For a movement serial# yes, for a case serial# no
The OP mentioned it to be a case serial. On solid gold SM there are sometimes peculiar case serial#, 6 digit or sometimes 5 digit.
So the case serial the OP mentioned may be OK

Yes, it would be normal.

My centre second example for instance:


If the 54696 number was on the outside of the caseback I suspect it was added by someone at a later time.
If it was on the inside of the caseback, it could be a casemaker's serial number (Omega did not make the cases at that time).

Here is a photo of the inside caseback of my reference OT2767SC from June 25th 1955.
The number 352 inside the "Poinçon de Maître" indicates that the case was made for Omega by Serva SA of La Chaux-de-Fonds, Switzerland.
The serial number is in the usual case number format (11,314,797) but other case manufactiurers could have used other number formats as mentioned by @mac_omega.



And here's what my beefy lug centre seconds looks like.

Usually the case serial## are 8 digit as shown by @JimInOz above.
But there are shorter serials on some gold watches.
I have a Constellation with a 6 digit serial# and I would not be surprised to find a 5 digit number