reply to u/KingCollectA at https://reddit.com/r/typewriters/comments/1rpr1ha/got_quite_lucky_finding_a_free_olympia_sg1_had_to/
Typically the SG1 has at least 3 serial numbers. Two matching ones on the main body (one hiding deep inside), and the third on the bottom portion of the carriage, which may or may not match the other two. (The carriages were meant to be easily swappable for machines with the same CPI/escapement sizes.) Removing the carriage will usually reveal the body serial number (typically a 7-XXXXXXXXX) format which you can compare with the grid of serial number ranges to see where yours fits in at https://typewriterdatabase.com/olympia.61.typewriter-serial-number-database.
I just got mine and have finished most of its servicing, though one or two small adjustments remain for it to be where I want it to be. Beyond this, it's been spectacular. See also: https://boffosocko.com/tag/Olympia-sg1/
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page from Wilf Beeching's Century of the Typewriter (1974 edition)