Well that narrows it down some.
If we have S2211xx as a model marked 28,, and S2214xx as a model 27-1 , we now know where to start looking for that 28-1.
I'm showing S223xxx as a 28-2 , so that narrows down the possibilities to about 2600 odd units.
No wonder no one has found a 28-1 yet
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Good luck and enjoy your H.P.'s Regards ,,Al