At first glance, the HP looks to me as a 9 mm Ultra. The caselength is (almost) the same as the comparing Makarov but I do not measure something like .720… On the other hand, there’s a significant difference measuring the rim: .369 for the HP and .389 for the Mak. So, Ultra or not? And does there exist a specific Ultra headstamp or is it simply 9mm?
I would call it a “9mm Police” round, also 9x18.
Here is a thread about them, showing the box. It is a 9mm Police, aka 9mm Ultra
So, welcome to the collection!
I believe that round was for a West German police pistol made by Walther in the 1970s called the PP SUPER. In the 1980s these pistols were imported into the U S.
I visited the SIG, Neuhausen factory in Switzerland at the time of development of the 9 mm Ultra/Police series. The round pictured here is part of the development, using a standard HP 89 mm Para bunter. While the round was developed to mimic the politically unsuitable (at the time) 9 mm Makarov round, the measurements are not base on the Makarov other than the case length. SIG had prototypes of the SIG-Sauer P230 on hand, marked as i recall with the caliber as .380 (perhaps actually marked 9 mm Kurz or the like). When asked about it, I was told that was to keep casual visitors from asking about the 9 mm Ultra project. At one stage of the development, SIG was handloading cartridges for testing on primed cases headstamped “GECO 9 mm Ultra.” They loaded one for me in my presence, Using A CNCS RN FMJ bullet with a fairly blunt ogive. . I was not a casual visitor by the way, having been invited to visit the factory by Director Hochuli (maybe spelled wrong) a year earlier when he was showing the prototype P220 SIG-Sauer 9 mm pistol to American jobbers and dealers, and visited out store.
When they gave me the general ballistics specs of the round, I asked in English simply “9 mm Makarov?” My Guide, one of the executives or engineers, sincply smiled and shrugged. That was worth a thousand words.
I still have the cartridge in my collection, along with somewhat over 40 other variations (case types, bullets, headstamps, etc.).
The best research work on this cartridge series that I know of is by Manfred Knothe, of the German Group of ECRA.