What do these circled 7's and 8's mean?

Question

I'm playing Schubert's 3rd impromptu, op. 90 D899. There is a circled number precisely under each trill, shown below, and I don't know how to interpret them.

I read here that they might indicate a change in the hand's position, but then

• I don't see why the circle notation would be restricted to trills (as the left hand has to do other brusque shifts) and, more importantly,
• it doesn't explain the circled 7 and 8!

Certainly the circled 1, 2 and 5 match the fingering of their respective notes, but how should I interpret the other two numbers? I don't think that they mean intervals as they seem too vague a notation for that purpose and they don't sound right to me.

Any clue on how to play these trills?

Answer 1

These circled numbers appear in the 1948 Henle edition edited by Walter Gieseking. They appear in numerical order except for no.1 which appears a second time between 6 and 7 (but over an identical measure). They presumably refer to editor's notes by Gieseking (and which are missing in the file on imslp.org).

The numbers are not in the newer Henle edition.