Um, holmburgers, curb your enthusiasm.

If you're referring to this http://kansascity.craigslist.org/pho/2097851094.html listing, I'm not sure you want to go near it.

Questions: is the 105 in or in front of the shutter? MP-3 lenses are generally in shutter (the shutter has a diaphragm, one cell is on each side of it). Most MP-3 lenses were made by Rodenstock and are in or on Prontor Press shutters; the longer ones (> 35 mm) are in shutter. If so, cable release only and another cable release is needed to open the shutter for focusing. MP-4 lenses all mount in front of the shutter and all were made by Tomioka. The MP-4 shutter is always a diaphragmless Copal #1 Press, will be marked MP-4 on the front, and will have an open shutter lever and a shutter release lever. Cable release socket too.

There is no MP-3 or -4 127/4.5 lens. 127/4.7 in Copal #1 Press for Polaroid Gelcams (CU-5, DS-34, and a few others), yes. This is a very good lens at all distances.

105 and 127 Tominons for Polaroid cameras are Tessars. The Gelcam 127 shoots very well on 2x3 at all distances (I have one, use it). 105s were offered in shutter (Gelcams) and for front-mounting (MP-4). I've never tried the 105, but since it is a Tessar it won't cover 4x5. Illuminate 4x5, maybe. Put good image in the corners, no way.

I've had all of the MP-4 Tominons but the 105. OK in their intended use, i.e., at magnifications >= 1:1, except the 135 which is poor. Lousy, as in I tried 'em, wouldn't use 'em, at distance.

Re brand, MP-3 lenses are usually Rodenstock Ysarons. Also tessar types and I've never tried any of them. Have tried the 35/4 Eurygon, sold it as quickly as I could.

Re "flat field copy lenses," nearly all lenses are designed to have minimal image curvature. Given the variable and often irregular curvature of the subjects we often shoot, only a real idiot would try to design and make a lens with anything but good flatness of field. Anything else would be useless most of the time.

With the exception of the 127/4.7 Gelcam lens, there are much better lenses at all of the focal lengths Polaroid offered. As I said, the shorter MP-4 Tominons I tried (17, 35, 50, 75) are all adequate macro lenses.