I believe the "F'" and "S" may have stood for first and second half of the year." Photo below : this H22 bass is from 1964 Photo below : this flat top guitar is from 1957.
Outside of a completed transaction, value has no meaning, so please do not ask me to value your guitar. Harmony instruments were produced on an industrial scale.
Even the less common models can't be considered "rare".
For example, on the photo below this guitar is without doubt a H22 (You can type a model number, with the H, on the search box on top of this page).
The first part of the serial number has no precise meaning relative to the date of production.
Its very weird and I'm wondering if there was ever any rumors of misprints in the serial numbers. - Yes, it's definitely not rare to see "misprints" serial stamps inside Harmony guitars, between similar models.
They were made on the same assembly line at the factory, and of course the body was the same before routing the holes.Ebay auction site is the main provider for rare parts, but you need to monitor the offers each day... You can check "tom69442" for repro pickguards, and "Harmony12X5" for some parts.For generic guitar parts and tools, well-known web sites include Stewmac and Allparts.It seems Harmony reset these numbers in every batch of the model, so a "6690H22" can be older than a "123H22" for example.Tip : on many guitars (easy to check on flat top acoustics), Harmony stamped the top of the guitar as well as the back, so if the number on the back is difficult to read, use a mirror and check under the top (near the neck) The other smaller stamp is the date stamp and indicates the year of production.From what I have gathered on your site this indicates that it is an Airline. They were used to keep together various parts of the same guitar on the assembly line.