Here's what I've found so far:
Oldest ones have big 3-tuning-forks in circle emblem on top, with YAMAHA below and JAPAN below that. Serial numbers begin with a zero?
Newest ones have no tuning-fork emblems, have YAMAHA on top, Established in 1857 in the middle and JAPAN on bottom. Serial numbers starting with a 3?
There appear to be two other types of markings: The second to oldest has a smaller emblem in front of the YAMAHA on the same line, followed by JAPAN on the line below. Serial numbers starting with a 1?
The second to newest appears to only say YAMAHA followed by JAPAN on the line below. I've seen serial numbers starting with a 2.
I still don't know when Yamaha replaced the YAS-21 with the 23.
Bill
Oldest ones have big 3-tuning-forks in circle emblem on top, with YAMAHA below and JAPAN below that. Serial numbers begin with a zero?
Newest ones have no tuning-fork emblems, have YAMAHA on top, Established in 1857 in the middle and JAPAN on bottom. Serial numbers starting with a 3?
There appear to be two other types of markings: The second to oldest has a smaller emblem in front of the YAMAHA on the same line, followed by JAPAN on the line below. Serial numbers starting with a 1?
The second to newest appears to only say YAMAHA followed by JAPAN on the line below. I've seen serial numbers starting with a 2.
I still don't know when Yamaha replaced the YAS-21 with the 23.
Bill