Thinking of that Everton friendly reminded me of this back in the 70s

[City123]

Mainz have only recently made it to the top division

They didn't play FSV Mainz, they played SVW Mainz

[Wales-Bales]

They didn't play FSV Mainz, they played SVW Mainz

So Mainz had two teams? The plot thickens ...

[Eric the Half a Bee]

SVW Weisenau-Mainz spent 4 seasons in the top flight (as it was pre-1963), FSV Mainz were ever-present. In 1967, SVW missed out on promotion to the Bundesliga, while 2 seasons later, both SVW and FSV were involved in a relegation battle. According to a translation of Wikipedia, FSV requested a merger with SVW, which was declined. FSV had the last laugh by surviving, while SVW dropped into the third tier, the amateur league (if you can call it that with 5 divisions comprising the second tier), where they would have been when they played Oxbarn.

Curiously, every online article I can find on the match calls Mainz a top division team....