I have a M260 set I purchased used. In speaking with Alex at Tele USA there are basically three different versions of this mic. There are two different revisions on the mic amp body motherboard, and there are two different tubes used by Tele. The Rev 1 board used the NOS Telefunkken tube (made by Ratheon) and the Rev 2 board uses a newer Phillips tube (because they ran out of the NOS Tele tubes). There are some sets that have the Rev 2 board and the NOS Tele tubes. This transition happened in about 2011. Alex called these the sweet spot models. Of course, if a Rev 1 model is sent to Tele for a new tube, it will get the Phillips tube, so I guess there are four different possible configurations.
I checked out my set with Alex (via serial numbers) and it turns out that I have a pair of the sweet spot mics - Rev 2 board and Tele NOS tubes.