I believe the "letter" designation tires were more toward the 1969 time frame, about the time that bias-belted tires were introduced. I have read of the repro L78-15s with either double or triple band whitewalls, but I don't know if it was Coker that was doing them or if it was somebody else. Many of the Cadillac and Buick people were excited about them, from what I've seen, plus the Imperials used the triple band whitewalls in several years (as I recall).
The L78-15 might not be the correct size for a 1955 Imperial, but it might be the closest outside diameter size to the older size that was OEM spec on that vehicle. Adding the bias-belted construction into the mix would probably be good too.
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