Maker: Diamond Medal.
The Diplomat was the high end of the Diamond Medal line, available in a full-size clipped version and a shorter ring-top. The dates given for the production run below are somewhat speculative; I’ve seen pre-Parker Diamond Metal pens with a tapered cap in the same mode as Sheaffer’s Balance line, and I make an assumption that the flat-top will be an earlier form.
Production Run: c. 1925 – c.1930
Cost When New: $3.98 (for modern value, try this calculator).
Size: 11.5 cm long capped, 14.4 cm posted, 10.0 cm uncapped.
Point: 14k gold.
Body: Celluloid.
Filler: Lever, capacity approx. 0.9 ml
Diamond Medal Diplomat
…as you can see from this.
If you are relying on the preceding information to win a bet or impress a teacher, you should read the site’s scholarly caveat. Remember, this is the internet, and it’s full of bad information.