Greetings from the Telefunken lab. I've attached some in-house reference frequency response comparisons between the TK67D (original center-tap) and TK51D (new edge-terminated) capsules. The black line is the TK67D, the blue/purple line is the new TK51D. To note, these plots are not absolute representations of the frequency response, but presented to highlight the differences between the two.
For part sourcing, we have our own sources for these parts and do not get them from any of the aforementioned companies. The design philosophy of the R-F-T series has always been utilizing overseas metalwork and capsules, combining them with quality components, NOS tubes and premium transformers.
Yes the capsules are sourced from overseas, but we spent considerable time finding the best quality sources we could, and test them to strict tolerance in-house prior to installation. Over the last year, we went through over half a dozen sources and numerous capsule models and options from each before deciding on the TK51D.
For the AR-51 specifically, we use the same Swiss-made Haufe T14/1 output transformer as used in our ELA M 251 and C12, a NOS R-F-T ECC81 vacuum tube, and custom-made polystyrene film capacitors in the US.
Every microphone is hand-built in our lab here in CT from raw parts to complete system. All design, PCB layout, and board plugging are also done here.
Hope that helps.