I bought an Old Mill Doyle a while back that has a similar printing flaw. On my card the period is not completely missing just very faint. The "Y" has the same portion missing. My guess is it was a printing plate issue as someone had pointed out earlier. I also have a Doyle with Polar Bear back that has the stem of the "Y" missing, making it appear as "NV". I jokingly call it the "Doyle, Nevada" variation.
One thing I've observed in collecting Doyles, there seems to be a much higher than normal rate of incidence with printing issues on the lettering. I guess this would have to be attributed to fixing the "N.Y. Natl" error on the printing plates.
My personal opinion is that while these cards can provide clues and insight into the mysteries of T206, they are not "variations" and not necessarily more valuable.
pictured- (top) Old Mill partial "Y"
(bottom) Polar Bear "Doyle N.V." version
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