Don't ya hate when someone asks a question, and then answers it themselves? Yeah, me too.
So ... I kept digging ... deeeeeeeeeeep.
On a very obscure , now defunct ( most recent post was '09) bbs I found a partial answer.
I thought, and was told, that the 735 was the upgrade of the 725 ..... maybe, but probably not.
The 728/729 series are essentially the some radio as the 725.
Someone posted they thought they were the same in '07.
I found a 728/729 manual on line and found the answer.
R238 ( maybe 239) which is under the bottom cover is a big, white variable pot .
The op manual for the 728/729 shows this as the side tone adjustment.
Same thing for the 725.