It's a governor to keep the engine revs constant while running for example a device connected to the PTO, in your case probably a fire fighting pump.
Set the revs on the hand throttle and the governor will try to maintain those revs whatever the load on the engine.
Usually parts of the linkage are missing so hopefully yours is complete.
You'll find some images of the setup on the internet
I have one on a Series I (I'll have to find a photo).
Colin
'56 Series 1 with homemade welder
'65 Series IIa Dormobile
'70 SIIa GS
'76 SIII 88" (Isuzu C240)
'81 SIII FFR
'95 Defender Tanami
Motorcycles :-
Vincent Rapide, Panther M100, Norton BIG4, Electra & Navigator, Matchless G80C, Suzuki SV650
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