I saw another frame on eBay and it looked like mine, with black paint under the red. I’m wondering if black primer was used. (There are multiple spots on the frame where there is black under the original red)
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You can see the red oxide primer under the black in your photo where the head badge was through a decent sized chip. Red lead is more of a brown tone color verses red.
And especially as your intent is to strip the paint down to bare metal it's really irrelevant what the original color is. That is, There's no difference between Schwinn's frame be it a Phantom, Hornet, Wasp and others except original color and name on the chain guard. . . I.E. any model.
As far as collectors go, the general rules is, it's your bike paint it anyway you want but do not expect that by doing so, the value is enhanced and or a collector is supposed to care what model type was designated, originally by Schwinn. I.E. it does appear there's evidence, in your photos, you have a red phantom B/C front post would be black too and the primary color would have been red with secondary black that was painted over that red.. but once repainted the only evidence is, it's a Schwinn frame that was made in whatever year the serial numbers say, every thing else is hearsay.
As far as the red lead under the red goes, I'm sure you'll find it somewhere and more than likely, where it does not look this way, the two are sticking together better than the red lead is to the metal.