Wow, thats a pretty weird looking circuit! Who designed that bit?
I assume Colin did. If anyone's got the original "hand drawn" circuit diagram that was in the early manuals it's probably got someone's initials on. Once you take out all the AC coupling components (R42 & R61, C5 & C37) it's just a normal inverting op-amp. The gain trim pot (5.1k pot + 5k resistor) is only there to let you adjust the gain when the gain pot is at the centre position.
R76 was added by Trevor Stride in the 241 (at least, I'm 90% certain the 241 was Trevor's), it's part of changing the gain of this stage (see below), it also keeps the pot wiper connection connected to something when the pot gets dirty and starts having bad connections when moved.
I think I've got this next bit right (after several attempts!) perhaps someone could confirm the 'T3' gains!!
The '01 has T2 with gain of 0dB and T3 with gain of -4dB (with pot at centre), the '241 has T2 at -4dB and T3 at 0dB so the overall gain is the same for both at -4db, but with 4dB more headroom in the '241 at low gains. That's the difference (and advantage) of the '241.
R42 & R61, C5 & C37 are there because the inputs of 5534s draw a lot (relatively) of dc current. If you connect them directly to pots then this causes a dc voltage to appear on the pot, then as you move it the slightest amount of dirt on the pot causes horrible dc "jumps" which comes out as crackles and bangs. So all over the 4k designs you'll see lots of extra electrolytics, then you need extra resistors (R42 & R61 here) so the op-amp can draw its input current from somewhere i.e. dc bias resistors. R42 is there so that the two inputs have the same bias voltage (i.e. same current through the same resistance) which helps keep the dc output voltage as near zero as you can get it. These resistors have to be quite large otherwise you mess up the gain in the rest of the circuit. C5 just keeps the noise down.
On the '291 inputs we sorted this out much better with a "dc servo" circuit e.g. IC14/C19/R39 (I say "we", that was entirely Paul F, I'd never come across them before), and then in the 9k channels I used dc servos everywhere.
Cheers,
Andy