Stupel, a valet, was one of the 45 retainers who accompanied the Romanovs into exile in Tobolsk.
Stupel is listed in the 2009 petition to the Procurator General of the Russian Federation for rehabilitation of all 45 retainers. However, the deposition given by Colonel Kobylinski listed Stupel as one of the nine servants the family had to discharge in Tobolsk after state funding of the prisoners was reduced. Consequently, he would not still have been employed at the time the remainder of the family would have been packing to leave Tobolsk.
Perhaps Marie either was not aware that Stupel had been let go, or she had forgotten. With 45 servants on hand and a valet typically serving only male members of the entourage, perhaps Marie did not have much contact with Stupel and had known his name and role but not much else about his comings and goings.
It's true that not much more is known about him than what you people have all noted above.
In her letters to M. F. Zanotti, Empress Alexandra does mention having to let him go, due to lack of funds.
However, the Imperial Family did give those servants a couple of months severance pay (from the Family's own funds) to support them until the ice broke and river navigation opened in the spring -- thus allowing them to leave town and return to Petrograd, etc.
So Stupel and the others were still in Tobolsk.
As for the letter from Steinberg's book (p. 300), we have a photocopy of it from GARF, and the attributions in the book are wrong. It was Empress Alexandra who wrote "I hope that Stupel will help with the packing" -- so maybe he was still around in some capacity, or could still be called upon to come assist the Children to leave. Obviously the Empress would know that he had been let go, since she herself had written about it earlier.
Princess Eugenie's book has a letter from Tobolsk to Ekaterinburg (May 1/14, 1918), in which G. D. Tatiana writes: "We haven't heard anything about Stup., and probably won't". That would seem to be in response to the Empress' earlier comment to them.
Pr. Eugenie's translator and she did not see the initial letter as a capital "S", so she put "stoup" in the French edition.
Of course, we have not seen the original Russian of that letter to be certain, but most likely it was Stupel whom G. D. Tatiana was referring to.
For some reason, in our records we have him listed as "Y. Stupel" (as in "Yakov" or "Yuri"), but I have no recollection why or where we would have found that initial. It may be simply a typo.
So, stay tuned...
I. N.