The first number at the beginning of a nine digit serial number is the origin code. A 1 means it was made in Germany, and the actual serial number is the next 8 numbers.
A 2 or a 5 is USA, 3 is Brazil and 8 is China. USA used up all the numbers for the 2 so started over with 5.
When I retired from STIHL Inc. in Virginia Beach in 2021 they were making almost 5 million serial numbered units a year, and now it is over that. Exports of almost half of those go to over 90 countries worldwide.
Magnus asked me when the origin code started. I don't know what year but I suspect it was around 1974 when Hans Peter Stihl opened the factory in USA and Brazil. So if your saw has less then 9 digits and is older than 1974 for sure it is a German built model, and the numbers may have been assigned in sequence by the assembly line for that model, or maybe not. I don't know for sure.
Regarding numbers being sequential, I know that when I started at STIHL Inc. in 2002 the number was generated by a computer as any one unit came off the line. So a saw might get a number, but a FS trimmer might get the very next number in sequence, and then a BG blower might be next after that. For example, when I get a box with 2 MS 170 saws or 2 FS 56 trimmers from STIHL, even though they came off the line only a few seconds apart, and went into a box side by side, the serial numbers may be off by 4 or 5 numbers, and then sometimes they may be sequential. What is weird about this is you might see a Tech Bulletin that says use a certain part number if the serial number is between a certain range. I guess the computer knows what range to use for a certain model even if some of the numbers were assigned to other models. The assembly line uses a BOM (Bill of Materials) which says that a carb or crankshaft or ignition module  with a certain part number is to be used during assembly, and then if an engineering change comes along saying to use a different part, the BOM changes and the serial number range that used the old part is now known and documented for the future. If a warranty part was called back from a dealer the part number that was being called back is known because of the serial number range and BOM. 
I posted this before I think, but here is a hand written document from 1959 for the Contra line:
[attachment=0]Contra 1959 serial numbers.jpg[/attachment]
So we see here the numbers were sequential and only had 5 digits. I have seen some of the conversations here regarding STIHL serial numbers and lots of speculation as to how to use them and we may never know for sure what method or reasoning was used back in the day.