[electronaut]

If I recall correctly, the EQP1R is a sort of "poor man's" version of the EQP1A, but it only makes a difference in the mid to high frequencies where the inductors are in the circuit.

One of the design goals of a passive EQ like this is to present the same impedance to the source signal at every frequency setting, while altering the corner frequency using different inductor/capacitor combinations. This is expensive because it requires a separate inductance value for every frequency, which means a big custom inductor with many taps at specific values. This is how the A version is made.

The R version compromises by using only two standard inductor values (50 mH and 100 mH) in order to get three values (the third being the two in series = 150 mH). With three values of inductance and a range of capacitors, you can get pretty close to the design goal if you give up a little in terms of maintaining the same impedance at all settings. This is a bummer for theoretical circuit-design nerds, but in the real world the difference is probably negligible for most things.