Here's the commercial version of the hydraulic fluid level switch. This one appeared on eBay. A Google search turns up several references to the same item. The key bit is the float switch in the reservoir cap. Otherwise any indicator light will work for the dash display. On 9/24/2014, Barry Measom in West Yorkshire, UK wrote: "I managed to find one of these Lockheed Warning Level kits in a toy shop! The cap is very similar to the plastic one fitted to replacement cylinders and doesn't look too out of place. The warning light is also a test switch to test bulb and circuit. The pinpoint contacts in the cap are plated but mine were badly tarnished and I had to carefully take the cap apart (the top is a TIGHT push fit) clean contacts and reassemble, but it now works fine. I mounted the warning light on to the outer dash brace, a little way down so it is out of site from outside of car but can be seen when seated in driving seat".

These photos are from an eBay auction in June 2015. Notice the kit includes the dash warning light with test switch function, and related wiring.