I just found out some more information about these horns.
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The 33 series is actually the non-polarized version of the much-more-common 34 series. Both operate at 0.063 amps
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There was also a 37 series, which was the non-polarized version of the 36 series.
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Because of this, as well as there being the 31 series (non-polarized AC) and the 35 series (polarized AC), only the 32 series horns (0.125 amps) had the “P” moniker in the model number for the polarized version.
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Wheelock continued to offer both pigtail and terminal versions until the end of 1985 / beginning of 1986; by that point, the pigtail models were finally discontinued. In the installation manual that I saw, the “T” moniker was in parentheses; so that’s how I figured this out. I would also say that most of the less common 30 series horns (32, 33, 35, and 37) were discontinued around the same time as the pigtail models. The 31/31T and 34/34T series were the most common 30 series horns, which is why the less common models ended up being discontinued.