Here is a rough overview of how to date K67/870 capsules installed in U67, M269, SM69, SM69fet, U87Ai, and K87 capsules used in U87 and U87i. (I will add photos later). The U67 and U87 types are identical in dimension, construction and acoustic design, except that the K87 uses a non-conductive spacer between the two capsule halves, and an isolation sleeve over the four capsule assembly screws, while the K67/870's spacer is made of conductive aluminum and the assembly screws are not sleeved.
Here then are the generations which are easily discernible even by non-experts.
1. 1960 to 1961. Similar appearance as early brass K47/49, but with two backplates mounted against each other with four M2 screws (if I recall correctly). Brass diaphragm rings, no threads in the backplates to screw the capsule into its mount. The U67 prototype, U60, used a clamping ring around the capsule; the first U67 production version was mounted with three tiny pointed set screws threaded into the mounting ring, similar to how M7 capsules were mounted to the U47 capsule mounting ring.
2. 1961 to 1966. Same as above, but capsule is held in the mount with four M2 slot screws (two are screwed into each backplate half)
3. 1966 to 1969. Brown fiberboard diaphragm rings replacing the brass rings
4. 1969 to ca.1980. First generation of plastic diaphragm rings (ivory color), whose 4 mounting screws were recessed into the plastic, to be flush with the ring's surface
5. 1980 to mid 1980s. Same as above, but with white instead of ivory-colored rings
6. Since mid 1980s. Same as above, but diaphragm mounting ring screws no longer recessed.
Versions 5 and 6 went through five generations combined; subtle differences through time are not easily noticeable by a casual observer. Likewise, it's initially not easy to separate a K67 from a K87 by looking at it. Upon closer inspection, you can see the 40μ aluminum separator ring between the two backplate halves on K67/870.
Since the late 1970s through 2009, all K67/87/870 are marked with a single digit, indicating the last digit of the year of manufacture (i.e. "7" for 1987, 1997, 2007, etc.) It is either marked with black felt pen or ink on the same ring with other production numbers, but in a separate space, or on the other diaphragm ring with no other markings.
Using a single number as year code can be confusing because numbers repeat every 10 years. But the combination of build characteristics and date number allows pinpointing the year of manufacture.
Since 2010, two digits are used to identify the year of manufacture. F. ex. '12' = 2012
© Klaus Heyne 2018