[jimonym]

Tim, I think you're probably right that the more subtle variations of the Ritchey card are the result of either reduced Dark Blue ink levels in the press or worn plates that didn't hold the ink as well as they once did. There are countless countless T206s out there with these slight differences in printing, though not as noticeable as the "doves" here. But the difference between pronounced "doves" and barely noticeable "doves" is a difference in ink levels. The difference between the presence of the "doves," no matter how light, and no "doves" is a difference in printing plates, I'd suggest.

I think the multiple plate theory answers a number of different T206 problems, but it isn't the explanation for everything. I do think it's plausible though that with Ritchey we could be looking at a 150 series card that appeared on at least two plates, one with the "doves" and one without -- and perhaps the one lacking the "doves" entered production first and ran longer than the "doves" plate. Then when the 150 series cards were repeated as part of the 350 series, the same two plates were run again, but in inverse numbers -- more time in production for the "doves" plate and less for the "no-doves." Or perhaps there was a new plate made, or several, having the "doves." That would account for the differences in populations between the two series.

Edited to add:
Keep in mind that in lithography each color requires its own printing plate, so the differences we're discussing here applied only to one plate (in the set of 9 or 8 or 10 or whatever the number was that created the image of Ritchey), the one for Dark Blue ink. All the other colors seem to be identical for both "doves" and "no-doves" examples. So it could also have been the case that all Ritcheys, whether they have the "doves" or not, shared all their printing plates but one, and we see a difference today because Ritchey and this sheet of T206s had an alternate Dark Blue ink plate. Hope that makes sense.