The most likely answer is that the larger number was the total number of machines produced by Singers when they only had a few factories and could keep up with and control the production output from Britain and America.
If you want to sell it, you may do better with a dealer (my first machine was from a dealer who dealt with schools and he had great machines) or to seek out the local group of reenactors, quilters, or cosplaying people.
Very early Singers from the 1850's up until the start of prefix letters in 1900 had two serial numbers.
There is a lot of controversy over why there were two lots of numbers.
There aren't any comprehensive lists for identifying models or manufacture dates for old BERNINA machines available online.
You can try visiting your nearest BERNINA store to find out if they can help, you might get lucky and find an authorized BERNINA Technician that's been around forever and knows all about the older models.