I love seeing corrected serial number stamps - it's a weird thing I know - and I'm always happy to get a machine that has one. In a way it sums up typewriters in general; oops, made a mistake, I'll just type over my error. 

As for the number itself, keep in mind that the Database is not complete or absolute. I have one '70s Facit with a serial number in the seven millions. And many other Facits used prefixes to indicate model type. The extra digit could be a digital model code, or just a change in the company's serial number system in the '70s.

I have seen some botched numbers that had a little more effort in the correction. On those the mistake was ground out before it was restamped, again much like typing errors but in this case going to the effort of using correction tape.

 

On Etsy...a Poland-made clone of what looks to be an ADDO machine.

Seller indicates its a clone of a Facit 1620, but the colors and the platen knob design makes it look more like an ADDO.

The typewriter, offered, has a wide carriage as well.  No Tab functions, at all, though.

https://www.etsy.com/listing/1268566801/lucznik-1303-typewriter-facit-1620-clone?ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_query=facit+typewriter&ref=sr_gallery-1-21&organic_search_click=1
.