From my factory work experience- serial numbers are like a filing system. Example- first two numbers are the country where is was made, the next two might tell which line it was made on, then others might be shipper, destination, area specific requirements (think Califonia kid pajama chemical restriction type thing)and sometimes one or two numbers actually might identify the worker who completed asembly or did the Q.C. Checks. Singer may have used some, all or none of these methods. Most factory made things are done like this for records, but after so many years, and the sale of the trademark, who really would know? I do know that car VINs are done this way.

Close, Kilbowie, which is near Glasgow on the banks of the river Clyde (Clydebank).

HA!! Oh that slayed me. I can see it all in my mind's eye...

Interesting! Anderson...where? In the US? I've never noticed an "A" on any other Singer model numbers but I haven't really been looking for all that long - is it uncommon? I knew about the "K", both of my 99's are "K's" and I've seen other "K" models, but I think I've only seen the "A" on the 500's, and I don't think I've seen a 500 without the "A". (But again...I've only seen a handful.)

Here is info on the Singer Anderson SC plant - and the A designation

http://www.singersewinginfo.co.uk/anderson/