Mahler 1, ending of 2nd movement

[jcopella]

3. Trompete (in F)

The last 3 bars are two written G#s (dotted halves) with a single trill notation spanning both -- with *double-sharp* indication below the trill mark, above each dotted-half -- followed by two grace notes (F-double sharp, G#) slurred to a written B, staccato eighth.

There's a notation in parentheses, "mit der unteren Note trillern" underneath the G#s.

I have this translated as, "Trill a step down".

My question -- is this a *half* step down or a *whole* step down. Taken literally, the German would (maybe?) indicate an implied whole step. BUT, the double-sharp marking underneath the trill line (and the concluding grace notes) seem to indicate a *half* step.

The weight of the evidence (to me) is that it's a half-step trill starting on the upper auxiliary between (concert) C#/C-natural, but because Mahler was so precise (and it seems he would've said a half-step, if he meant a half-step), it makes me second-guess my instinct.

Esteemed orchestral colleagues, what say you of this apparent conundrum?

Edit to add: maybe the issue is my translation, which is from the Gorham book of translations of directions in the brass parts of Mahler. Google Translate thinks this means "trill with the lower note" which (I think) would support the half-step interpretation -- C# to B-double sharp, starting on the upper note.

[mm55]

Yes, it means "trill with the lower note", and the double-sharp indicates the accidental on that lower note.
_________________
'75 Bach Strad 180ML/37
'79 King Silver Flair
'07 Flip Oakes Wild Thing
'42 Selmer US
'90 Yamaha YTR6450S(C)
'12 Eastman ETR-540S (D/Eb)
'10 Carol CPT-300LR pkt
'89 Yamaha YCR2330S crnt
'13 CarolBrass CFL-6200-GSS-BG flg