Is it safe to assume that the shades of the ultramarine of the First Issue U.S. Stamps should match with the ultramarine of the Private Die Revenue Stamps? I have seen certified RO83, RO148 and RO186 stating they are the ultramarine varieties that have nothing in common with any of the First Issue Revenue Stamps except the blue shades of the 2cent Proprietary similar to ultramarine. Another Match Stamp that is commonly mistaken as ultramarine is RO173, these are stamps from a worn plate. This stamp in the true ultramarine is easy to distinguish. I read an article stating the RO17 stamp doesn't exist in the ultramarine shade, has anybody seen one? Scott has a listing for it every year and I have not found one sold in any previous auction. I hope you collectors of Private Die Stamps can provide additional information regarding this subject.
Even among the first issues, not every ultramarine stamp is identical. These were hand mixed shades done in daylight or candlelight, and leftovers from one day would be mixed with the next batch created. so there is always a certain amount of variation. Yes, RO17u exists. It is a very scarce shade; catalog prices for M&M's do not always reflect true scarcity. Some are popular and cat higher then the scarcity level and some are much more difficult to find and cat very little because they are not popular designs. Still others are very common to fairly common until one wants a nice example with perfs on all four sides. THEN one has a real hunt.