I've been doing some listening and some experimenting and I have come to the conclusion that Bruce's rhythm sound on the Strat is not at all the same for each song and that he must have used a lot of different settings, both on the amp and on his guitar. Why shouldn't he if Hank did? For example, the rhythm for Midnight is played on a Strat even on the original recording and has a sound with very pronounced mids. The same is true for Cosy. Many other songs were also recorded with Bruce using a Strat, like FBI. So, when playing live Bruce would have had many different settings for different songs, at least if he wanted to sound anything like he did on the recordings. Since he played on an AC15 and later an AC30, he would have had to eliminate the bass and perhaps even the treble to a large extent, very much in the way that Hank did, so as not to get in the way of the bass, but he also had to cut down the treble to eliminate any shrillness, because he basically needed a mellow acoustic tone, and then he would roll down the volume, and with the middle pickup he would sound a lot like an acoustic. Changing the pickup to the bridge/middle position and just slightly rolling down the tone pot, he would get a very authentic Savage rhythm sound. For FBI he would have to boost the mids again and get a sound much like Hank's . . . and so on. Chances are that later Bruce also used some discreet acoustic pedal . . .
Since my main "position" has always been rhythm guitar, I always wondered why Bruce, who is such a perfectionist, would just put his pickup selector in the middle position and play away. Now after trying out many different combinations, I'm pretty sure that he too went to some lengths to get the right rhythm sound for every Shadows song when playing live . . . hence the "trade secrets."