These are supposedly the manufacture date of the electrical harness or pickguard assembly, by Rowe Industries, but should give a close guess to the year of production of the guitar itself.
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.
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".
Either they re-used a back prepared for the previous batch, two years before on the line, or they mistakenly used a 1961 stamp in 1963.
It looks like Harmony serial and date stamps were meant to manage the different parts and instruments on the production line, not to be used as reference by collectors 50 years later... - My brother has a Harmony Meteor in great shape but the the serial number is 84H8448. Do you think someone added the Meteor Label on the Headstock? - These stamped numbers inside Harmony made guitars were not intended as real serial numbers.
They always are on a place easy to read, ie near the f-holes on a archtop.
The larger one is the serial number, and the part after the "H" letter is the model number.
- If your guitar is an electric, most models produced after 1960 (and all cutaway) have a laminated top and/or body.
Older "full box" archtops designed from acoustic models still were made from solid woods.
Ok, there is some rare exceptions to this rule, for some very late models produced a couple of years before the close of the factory in 1976.