At least 2 very early 55s are known with absolutely no theft proof number, and there is another small group, less than a half-dozen, that have theft-proof numbers in the 1936-37 range. I've always assumed this was just indicative of the total chaos that likely prevailed as Packard re-assumed body manufacture and at the same launched a nearly completely new car line.
All the 1956 models are "D" prefix, 1955 cars except as noted above are a mix of "C" and "D" prefix. Many folks read the prefix "D" as an "O"; the dies were apparently getting pretty worn by then and the stamped characters are not as crisp as in earlier years - but if you look carefully at the corners you'll see the left-side corners are more square, thus a "D".
Here's the last-known update to the style blueprint. The "D" was somewhat more square for 1955/56, perhaps there was an updated blueprint and revised dies but I've never found any record of it.
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