First, comparison between similar mics shows what sort of repeatability you can expect after tinkering with your microphones. In general, it’s pretty good. Here are the four 991s after mod.

They are all close enough to serve as stereo pairs or a quad if you’re feeling exotic.

Here are the four 990 side address mics. A bit more spread here, but general character remains.

Comparing the 990 to the 991 reveals the effects of the body shape and head basket. Yellow is the 991 pencil mic, green is the 990 side address body with inner layer of screen removed. Advantage to the more open pencil style. Same capsule design and modification in each case.

Finally, two of the 990 capsules had the acoustic resistance plates (the clear plastic ones with the eight small holes) drilled out so the holes were no longer effective, the small brass washers were discarded, and a cotton felt washer the diameter of the body was installed between the brass backplate and the opened-up resistance plate. The capsule was tightened up to compress the cotton by about 50%, to 1/16 inch. The felt serves as a new acoustic delay and low-pass filter. It offers a slightly flatter response in the 990 body. Yellow = original, green = felt.

The felt capsules haven’t been tried on the pencil bodies yet.

The felt delay washers were a work-around after an experiment drilling out the delay plates to see if the capsule would become a fig-8 pattern. It did, but the diaphragm tension and backplate tuning were too far off for my purpose (sounded too weird on too many things), so I reverted to cardioid pattern. Further experiments with various materials and amounts of squeeze may follow. The fig-8 config did sound nice, warm and crisp, for male voice-over.