Georgioh, in the "golden age" Levinson (Madrigal) period, there were the 20 (amp) 23 (amp) 25 (phono pre) 26 (preamp) and 27 (amp).
Improvements were made to all of them kicking them into major new orbits, sonically. New type circuit board material, teflon capacitors and metal film resistors, improved power supplies (in the amps) etc.
The new designations (respectively) were the 20.5 and 20.6 (amps) 25s (phono pre) 26s (preamp) and 27.5 (amp). These are all still very much sought after for their extreme quietness and almost tube-like musicality.
I had a 23 and then a 23.5 and the difference was quite remarkable (I still have my 23.5 after 15 years!)