3c Green On Blue

I've come across this blue cover. It measures 139 x 84mm. The back of the envelope is blank and it has a watermark (unsure of type). I believe its a plimpton but my knowledge on covers is scarce. I don't have a catalogue and any help would be greatly appreciated.

I'll also post a couple of other covers that have the same addressee. These also measure approx 140 x 80mm.

mc49, it's difficult to tell from the photos if these are Plimpton or Reay Issues. The difference, according to Scotts being the Plimpton are inferior to the fine engraving quality of the Reay.
But judging by head shape, I think you may be correct. Notice the centre image has a rounder head than the other two. The U44 has quite a round dome, and the numerals seem finer. This was issued on both cream and amber coloured paper. Again hard to tell from the photos what colours you exactly are. The other versions U45 and U46 have slight differences. But they were printed on various colours including blue, amber, cream, fawn and white.
Hope this helps.

Thanks James,

I did not think of looking at the head shape and I think I may have sold the riddle. In order from top to bottom - the blue 3c is a plimpton U166, the second is a plimpton 161 and the final is a Reay 83 - however I could be wrong.

A little biographical information on the addressee on your covers: E.S.P. Hine (1838-1925), a native of Gibson, Pennsylvania, was a professor and school teacher in Harford Township Common Schools (Nicholson Independent School) for four terms beginning in 1872.