John W., the capsule in the TLM 103 isn't the same as the one used in the U 87--the K 103 has only one diaphragm, with an interesting "second backplate" arrangement in place of the second diaphragm that a U 87's capsule would have, and it was designed for a different, and less costly, method of assembly. But its acoustical design is based on certain parameters of the U 87's capsule, and the family resemblance in the sound of the two microphones is unmistakeable, particularly the midrange.
However, the U 87 has circuitry to tame the capsule's high-frequency emphasis while the TLM 103 does not. If a cardioid pattern is all that's needed, I'd agree with the advice to try the TLM 103 as a less expensive alternative, but the user might want to consider some EQ to reduce the TLM's 3 to 4 dB rise in high-frequency response. Or not--it can be very sweet on some material, while on other material it can be "gilding the lily."
--best regards