The 'optional' factory hour meter for the B-7100 was a 12V powered and counted up accumulated operating hours only and did not register engine RPM's.
Your B-7100's engine should have a cover plate bolted to the front of the engine, covering the location where a Kubota Tach Drive Unit (67111-58810 and it's gasket 15221-88130) could be mounted.
The correct cable driven hour meter that will fit your B-7100 is part # 67800-55400, spacer # 67800-55420, and the cable is part # 67800-55410.
Now here's the thing I don't know.
I don't know if the factory 'cable driven' hour meter has a tach needle that wil register engine rpms like the oval shaped early L-Series tractors had.
Those meters registered engine rpm's and also racked up accumulated engine operating hours. Many showed a mark on the tach scale indicating the correct engine rpm for running the PTO at 540 speed.
To confound things even further, my own B-6200DT is fitted with the factory tach drive unit on the engine, has no tach cable, and on the dash it is fitted with a factory 12V Kubota hour meter!
Can anyone upload a picture of a cable driven hour meter from their B-42/52/62/72/82/9200??
View attachment 2845
The tach drive units on the 3 cylinder B Series engines were all the same.