From one of the best private collection's of NOS Mercedes parts visit my website- hmcmercedes and add dot com at the end comes this....

Mercedes ATE Tandem Master Brake Cylinder REPAIR KIT 03.0370-2923.2

This has a Piston part number of 3.3312-2309.9

I also have one not listed with a Piston part # 3.3312-2309.1 , let me know if you have interest in that one if there's a difference.

Condition is NOS in the tube to protect it along with the instructions.

23.8 mm bore

Check your model for fitment as it works with many including the W114+ W115