A Very Brief History of Men’s Swimwear.

[midlifeswimmer]

The 1970's Speedo 100% nylon suits came in both 4 and 3 inch sides. The 4 inch side ones had five-digit model numbers in the 70000 series, the 3 inch side ones were in the 72000 series. 70001 and 72001 were the solid color model. Then there were various sewn panel variations.

[Minimalist75]

The Speedo Solar must have arrived later.
It had a 1" side seam.
Rumor had it that one of the advantages of it was that you could tan in it and there was little chance of any of the untanned area showing when wearing a 2" competition brief.

[Torchwatch]

Here in England we were never offered the Speedo Solar or the Speedo Thong, but before the introduction of Lycra we had a choice of 3" and 2" side designs in nylon.
Getting 3" black nylon Speedos was an adventure and then to discover they do them in 2" wow!

[Minimalist75]

In the US, Solars were easy to find, thongs less so.