Hiall,
I have a an old Holton double here and was womdering how to reverse the 4th or "change" valve.

It's not like a Yamaha or other that has another screw hole on the other side of the stop arm.

Can you "backwards string" this thing? I tried but with bad results.

TIA!!!

No, you can't change it.
On Holtons the horn will stand in F.