An A15 bulb — also known as an appliance bulb — often has the same size screw base as an A19 bulb, which is another reason you might think they are interchangeable. Base size for A15 bulbs isn't universal, though; some have an E17 screw base, which is known as an intermediate base...
Following that logic, there are perhaps up to 15 sets of A, B, C, and D. Now one would presume that the first batch (#1 to 500) would be etched 'A1; B1; C1; D1' and the last batch (#7000 to 7500) would be 'A15; B15; C15; D15'. Strangely that is not the case for my...