And of interest...
73 is the 21st prime number, it's mirror 37 is the 12th prime number, and it's mirror 21 is the product of 7 and 3.
Wow, go figure...
The usage of "73" comes from the Phillips Code, originally devised in the era of telegraphs to speed up transmission of common messages by mapping them to numbers. The numeric code is a small part of the abbreviations outlined in the Phillips Code (developed by telegrapher Walter P. Phillips).
73 -> Best regards
With phone modes SSB, FM etc. "Seventy Three" seems to be the norm.
Sometimes you’ll hear 73 expressed as “Seven Three”, which corresponds to how the Morse characters were sent. It is incorrect to say “Seventy-Three’s” since this would literally mean “Best Regards’s”.
73 DE Bruce