No, they are not identical - but very similar! We have build the "M 3500" exclusively for Revox, based on the regular M 201N(C). There were some differences: "M 3500" had five vents behind the capsule, the former M 201N(C) had six (the current M 201TG again has five). "M 3500" had a 600 ohms voicecoil while M 201N(C) uses a 200 ohms coil together with a passive filter which tames the presence area of the response. Therefore, the "M 3500" sounded a bit more "forward", more "in your face" than the slightly darker and flatter M 201N(C) (and the current M 201TG).
We do not have any spareparts for "M 3500" left in stock, but in most cases, we can offer according M 201N(C) / TG parts!