Colin - Looks like an aftermarket solid state cutout.
A couple tests:
1 - With the engine running half-throttle or faster, jumper the two cutout posts together with #12 or heavier wire (expect a spark when you connect!). If the ammeter shows a charge with jumper connected the cutout is shot.
2 - Disconnect both wires from the cutout. Now take a VOM or DMM and measure the resistance between the two posts in both directions. A good cutout will show infinite resistance one direction and nearly 0 resistance the other. (If resistance is infinite (or 0) in both directions the cutout is bad.)
Your OEM wiring looks pretty tatty. A new wiring harness - either purchased or home-made - would be a good investment!