denababy Thanks for that, if only I could read German.
I could tell from just the pictures even though my German is rusty :-).
The bolt is 3/8", the slot in the hockey puck clears the azimuth pillar, and the hole under the hockey puck is tapped for M12, according to the Technical Data page.
If you don't read any German at all, there are two icons showing a little German flag and a little split UK/USA flag near the top right of that web page that lets you select the language, to allow you to order. Your credit card company will do the appropriate $ to € conversion when you are charged.
FWIW, my William Optics Mortar mounting plate also adapts directly to the 3/8" hockey puck that RainbowAstro provided in their kit. However, I drilled the W.O. mounting plate to take the two M8 holes that are directly at the base of the RST-135 to get better stability, instead of going through a single 3/8" bolt.