Ty Ford wrote:

We all pretty much know that the U 87 and TLM 103 don't sound alike
for [ma]ny reasons. So the thought that one "is based on the other"
becomes more relative, low order explanation than an accurate technical
description.

This was discussed in some detail, with contributions from Karl Winkler
of Sennheiser/Neumann and (ahem) some from myself, in a thread on this
newsgroup in September, 2000. Unfortunately the thread got split in two
when someone altered the subject line. The relevant part can be seen
on http://tinyurl.com/2jjj7 (the original question was about "InnerTube"
electronics for the U 87, and can be seen on http://tinyurl.com/38fog).
See in particular the reference to the model "TLM 171" in that thread.

Upshot is that the K 103 capsule is similar in design to 1/2 of a K 87,
but single-sided and thus cardioid only. (I believe that its process of
manufacture also involves fewer manual steps than that of the K 87; it
is considerably less expensive.) But keep in mind that the electronics
of the U 87 family of microphones roll off the top end response of the
capsule by 3 - 7 (!) dB depending different on the particular model.
The TLM 103's electronics don't use any such rolloff.