I'm new to this, so bear with me. I had always supposed that the Mazda S-1 and S-2 sunlamp bulbs were immediately followed by the RS reflector sunlamp. The other day at a thrift shop I picked up a Sears suitcase style sunlamp that, judging from the graphics and construction, dates from the late thirties. With it were two Mazda S-4 sunlamp bulbs, one by GE, the other by Westinghouse. They are of very heavy clear glass, A-21 envelope, Edison admedium base, and contain a small quartz arc tube, starting resistor, etc. The GE has had some use, and shows end-darkening from electrode sputtering. The Westinghouse seems almost unused, and has nicer construction than the GE; pretty mica insulation plates, and a higher-quality resistor. Both bulbs operate well. I have searched old advertising, company literature, and can find absolutely nothing on these bulbs. Are these bulbs perhaps a short-lived step between the S-1-S-2 and the RS, or am I overlooking something? Was there an S-3 sunlamp bulb? Any help at all would be appreciated. I will send a picture when I can get hold of a digital camera. Thanks...Dave Salny