The image below a shows a 7" Mexican release of Abba's "Fernando" (with "Mamma Mia" on the b-side which seems like a very odd choice) from my collection.
The single was released in Mexico in May 1976 where it reached number 1 in the English chart - a position it kept for 5 weeks. The single was part of a run of five number one singles in a row for Abba on the chart ; "S.O.S", "Fernando", "Dancing Queen", "Money, Money, Money" and "Knowing Me, Knowing You".
Although the next single they released in Mexico "The Name Of The Game" only managed to reach number 10 ; however the following single "Take A Chance On Me" returned the group to the top of the charts.
You would imagine that if the record company had released "Mamma Mia" as a single on its own rather than as a b-side it would also have reached the top of the charts.