With in the mood, I can play the opening 8 measures with no problem,–it’s perfectly in tune with your backing track, but it’s when I get into the Arpeggios starting on the 9th measure (Tenor) where it goes D-F-Bb, changes to E-G-Bb, and then F-A-C, there’s something about the rhythm of the song there that I can hear but playing it is a little different. Same thing with these same licks/melody of these Arpeggios at the end of the song. It’s all got a certain swing/rhythm to it.
The weird thing is that I’ve learned how to play on the Tenor “Puttin on the Ritz” as performed by Robbie Williams. It’s a really neat song! There’s a few Arpeggios in the middle of the song but no where near like In the Mood. I had to work at it a little but I got it–I’ll upload it soon so you can see what I’m talking about with these basic Arpeggios this song has; my recording equipment from Amazon will be here this week. The Bb key I used in that melody was the B with the small key right next to it and trying to play it with the side Bb key was a little hard for me. I’ll try one of the other 2 Bb fingering options like you suggested and see how it goes,
One’s thing for sure: Glenn Miller and his band sure knew their music–for them to be able to play those Arpeggios, all together in unison and in perfect harmony; that’s amazing.