1610c hasn't moved in valuation. It's consistently been at this level. I think the buyer was a little trigger happy on this. Hipstamp regularly offers sales, particularly on high prices stamps that could have helped the buyer realize a total reduction in cost. He didn't face any competition.
As far as price it's fair. I believe it's because they come up for sale on a regular basis and satisfy demand.
The question I have is, is there a catalyst that could send stamps like this higher, now that we're 40 years later?