here's a photo of the auditronics mic input (left mounted on the pcb), je-6110k-a pc (center), and a utc ouncer (right) for size comparison.
i wouldn't say that the je-6110k-a is smaller than an ouncer, just shorter. i would even guess that the core size is the same, just horizontal instead of vertical.
i've seen a taller can version of the same mic input, also painted yellow, but the core inside was the same size. they probably changed over to jensens later in production, but i don't think that's what these little yellow guys are. they don't look like any utcs i've seen either.
for inductors, i don't know of anyone who makes off the shelf exactly equivalent replacements. you can get some samples in similar values from wilco, the HFT series of toroid inductors:
http://wilcocorp.com/catalog/series.cfm?series=HFT
if you're willing to order enough they'll probably make them in the exact values for you. you might also ask at mcemagnetics.com, but they'll probably be too expensive.
for the 20mh and 75mh (or in your case 100mh), you could try the toko or fastron inductors. they aren't the same thing, but they might work okay. the bigger values are really hard to find in off the shelf inductors. consider winding your own if you only need a few.
if anyone has any more questions, i'll try to answer them.
ed
ps- dave and anyone else, i stopped working on redrawing the eq section of the 501 by itself. if i ever go back to it, i'll post it here for all to see.