The construction of this sub looks very similar to my ancient (35 plus years) Krick Type 1A sub.
I.e. wooden hull made of a sandwich construction of four thick planks hollowed out for the payload. See attached pics of the interior of my Krick U25 built on that principle.
Which leads me to suspect that this is a surface runner or dynamic diving only model, like my U25 (up to now😉). I.e. no dive tank, model must be forced under with the dive planes while moving forward at speed.
What I miss though in your pics is a sealing gasket!?
1 Do any wires go from this mysterious module to the motor?
2 Is there anything lying about deep down in the hull (as low as possible) resembling a pair of metal electrodes or a resistor? Probably close to the prop shafts and/or the breakthroughs for the rudder and dive plane control cables.
My first thought is that this module is a combi ESC and Water Intrusion Alarm module.
The ESC section controls motor speed and the Intrusion alarm warns of leaks, e.g. through prop shafts or control cable exits, and perhaps should be connected to a Fail Safe warning circuit. E.g. lights and/or audio alarm if a surface runner or automatic surfacing if a diving model.
A search for Garron Micronics throws up many interesting scientific / physics links but nowt to to do with model subs🤔
A word to the wise!
Please try to post photos in the original format preferably jpg, not screen shots of a viewer!
Also try to ensure that any text / legends are readable without getting a crick in the neck or a slipped disc!
Had to download and convert your pics (in Irfan View) to make any sense of them!
Any other pics you have of the whole sub may help to solve this mystery👍
Where did it come from?
Cheers, Doug 😎