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On some of the older gauges, there is radioactive luminescent paint on the numbers and the dial pointers. I can only speculate on what was used. It could be radium, promethium, or even tritium paint. In all cases, it is harmless as it was deployed. There is some risk when the glass gets...

Mostly true, but somewhat simplistic. Alpha particles are just energetic helium nuclei. Beta particles are just energetic electrons, and gamma particles are very energetic photons. If an alpha, or a beta emitter gets inside of your body, all of the particles will be dissipated by your body...

Unfortunately that doesn't always work. The old radium dialed gauges were so hot that the radium burned up the phosphor material that did the glowing, and they stopped glowing. -Chuck

The radiation comes from vermiculite used as a refractory. It is harmless. You run more risk from the asbestos. -Chuck

So, it would appear that if you have an SW-503A speedometer, it is radium. I have tested my SW-503C and it is definitely not radioactive. Safety wise, the radiation is not really a problem. It decreases rapidly as the distance from the gauge is increased. Get a couple of feet away, and you...