Release dates are approximated (at least for my own research, and within TBM / BTBM) by copyright certificate date whenever other previously published source material did not yield any significant correlation in conjunction with the date noted on the certificate.
Otherwise, standing alone without deadwax, industry magazine review, ad or paperwork from the session, or corroborated against other print source material from the time-frame, a copyright noted date is, in my own experience, reliable at least 90% of the time. It is a reliable approximation to note that a release falls within the calendar month noted on the certificate. I have only noted a handful of "late" filings when cross-checking, one being for "Searching" by the Omens, that song was not filed in 1966, but much later.
George G., Jeff L. Tom F. and Bill N. were among, if not only, the first knowledgeable collectors I was aware of to study in detail the codes and numberings we now call "dead wax" details, or vinyl trail-off etchings, stamps etc. Some collectors who George used to label as "Joe Rock & Roll" made fun of and snubbed the 'nerdery' of this effort, lumping these detail oriented researchers alongside the Beatle nuts who were more interested in a matrix number than the music.
Well, George and company had been collating all of this date for many years by the time I came into the picture (1981) as a collector. I eventually incorporated their results with my own findings. I had intended to publish the "tables" I created via mathematical computations for the numerous pressing plants active in the 1960s, but some still need more fine-tuning by examination of other 45rpm genres I did not document (country-western, easy listening, etc.). Yeah, there are blogs and such with dead wax codes out on the internet, but they are not too reliable and off by a year or more in numerous cases. 45cat is attempting to catalog this data and is far more reliable to reference than the horrid and erratic discogs, if that is what you are seeking.
To answer gregk25 - EP copyright numbers refer to published material. There is a section on the certificate that specifically refers to "Date first made public" on it, and the person who fills out the form is required to provide the exact date. This refers to the release date of a song or the date the song was first 'made available' . EP copyrights are filed separate from the vast EU numbered volumes.
EU copyright numbers refer to unpublished compositions. Technically this should define anything that was written and then submitted prior to recording, etc, but this is not the case at all. If something was made commercially available after filing an unpublished composition, the person or entity that filed the unpublished copyright needed to submit the composition as "published, thereby acquiring an updated copyright status of EP instead of EU. Such instances are not common, however.
