USA v UK 45's

[Stax_of_Motown]

Not sure if this was universal, but what was the reason USA 45's were undated yet UK ones showed year on the label? Google didn't have any answer on this.

[Motown Eddie]

That was just the industry practices at that time. Records issued in the UK started putting a "P" along with the year a record was released years before it became a practice in the US. Most record companies in the USA didn't start putting put the year of release on most of their records until around 1972 [when there would be a "P" along with the year of the release and the name of the copyright holder on the record's label].