Serial number
The dashXX denotes a series. For example -27 would be E series engines, some of that series have different front main bearings that do not support a constant speed prop although the case can be converted during overhaul. An A after the dash number denotes wide deck, dash 27 without the A would be narrow deck.
I have heard that some of the very early 0 320's are simply 0 320, no prefix or suffix. Those examples were most frequently installed on 1955 model year Pipers, some of which were likely manufactured in late 54.