As a result of your thread, I just checked 12 postally used (but uncancelled, of course) examples of the above-referenced stamp as had been received on various bulk rate mail. Five (5) examples are the easy-to-spot Scott 2606. The remaining (7) are 2607 (I think). I don't have the 2608 (yet).
Frankly, I never really took much notice of these varieties until you pointed them out.
[Edit] Just found a few more of these stamps and I think I have the three examples designated by Scott Number 2606, 2607 and 2608. Does this look correct? The last two look similar but there is a distinctive color variation between them, as the last one (2608) is defined by the Scott Catalog as "violet blue":
