I made a short video about that dial tester in function.
Gossen (now: Gossen Metrawatt) is known for professional testing equipment. Photo exposure meters were a consumer product made by them for years.
This piece of kit was pre-owned by the (West-)German armed forces (Bundeswehr) and made 6/66. it looks like it was rarely or never used. It has a calibration sticker with a barcode, so presumably it was serviced for a while...
I found it on eBay as "defective" but gave it a try. There is documentation available on the web:
https://www.wasser.de/telefon-alt/datenbank/inhalt-download/d1001175/Aleitung.pdf (https://www.wasser.de/telefon-alt/datenbank/inhalt-download/d1001175/Aleitung.pdf)
and also pictures.
Here's a short video I made of it in use: https://www.youtube.com/watch?v=TWfaOfDdpfs (https://www.youtube.com/watch?v=TWfaOfDdpfs)
Setting I (or rather "J" on the meter) is the make-break-ratio, should be within the red marks for German standard 60:40, Hz is the pulse frequency (10 pulses per second). There is a selector switch to measure both values, and a calibration knob that has to be pressed and tuned to max. readout.
(https://www.wasser.de/telefon-alt/datenbank/inhalt-bilder/bb1001168.jpg?1647721476)
(https://www.wasser.de/telefon-alt/datenbank/inhalt-bilder/bb1001171.jpg?1647721511)
Ofcourse yours are more modern, but it seems definitly more easy to use than mine. Looks practical!
http://www.classicrotaryphones.com/forum/index.php?topic=21462.msg218785#msg218785
That was why I wanted it despite of the relatively steep price (33 € on eBay)...
I have the pulse logger too, and it did cost me less money ;) but like you say it is awkward to use and the paper rolls are no longer made, so a shame to use them up.
I have not seen many "Gossen" testers from the Post, but apparently the military hoarded them. This surplus shop (https://bw-schmitti.de/produkt/gossen-nummernschalterpruefgeraet-mit-zeigerfrequenzmesser-typ-nspg-z-65/) has 17 of them for sale, but at a price!!!
It uses a 3 Volt "2R10" battery that used to be common for small torches, but no longer is. I soldered in a 3 V lithium cell from a defective smoke alarm (also available as "Photo special" CR123A)
I have seen pictures of a version with a vibrating reed frequency meter.
Edit to add: The above pictures are links from the Wasser.de website. The video shows my actual object.
Thanks for sharing. :)
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Screenshots from link:
I have bought an adapter from USB to RS-232 and use the software described here in this thread: http://www.classicrotaryphones.com/forum/index.php?topic=21454.msg218706#msg218706 (http://www.classicrotaryphones.com/forum/index.php?topic=21454.msg218706#msg218706)
Easy to use and pretty accurate. Also interesting reading on these pages in German, but google translate makes it easy to understand. :) https://elektronikbasteln.pl7.de/nummernschalter