[JT118]

Good link with many doc and youtube video etc :

https://www.youtube.com/channel/UC1x0s3q...G7NK_613Zw

Watching this I am sure your gauge is a: OTA Tipping Bucket Rain Gauge
OSC971OB100A ( A = 0.2mm per tip rather than the 0.5mm per tip setup) not a hydrological services gauge. It is just possible, though unlikely, that you may have a 0.5mm rather than 0.2mm bucket. Hence the need to check using above methods. You are correct in your measurement of 200mm so you definitely need to use 628ml of water as your standard test and this should register 20mm on readout. You can see in your photos the rivet holes where the makers plate has been removed.

http://www.meteoew.nl/ota-rain-gauge.html

http://www.osc-tokyo-japan.sakura.ne.jp/...OB100.html

OSC971OB100.pdf Size: 235 KB  Downloads: 350

https://www.youtube.com/watch?v=4FAtUuULfHc