I'm no specialist in this particular issue, but according to the Scott Specialized stamp catalog there is no "deep violet" color shade known for this stamp. The choices are violet (most common), deep red violet (very rare), light reddish violet and red violet.
Although monitors can be deceptive is accurately identifying color, I don't see where the above scan is anything but violet. If it were any of the red violet varieties, the color would be much more distinctive.
This commentary might interest you, where they suggest that even some subtle changes in storage and/or chemical treatment can create the appearance of the rare color variety:
http://www.ebay.com/gds/THE-US-VICT...45780/g.htmlOn the other hand, Siegel does refer to a "deep violet" color rather than a "deep red violet" color in describing the "rare" 537a. I'm not sure why they vary from what is stated in the Scott Catalog, but all sources seem to agree the only way to really tell is to get the stamp expertized -- yet even the experts can't always agree.
http://www.siegelauctions.com/dynam...2020,%202012