The earliest speed loader I can recall reading about was introduced in the 1930s by the Revolver Cartridge Clip Corporation. It used six metal clips in rows of two each to hold the rounds in position to drop into a K-frame .38 revolver. Once they were chambered, you pulled the ring on the side of the unit sideways to pull the case heads out of the clips and leaving the cartridges in the chamber. There's now a modern rubber version of it, although I don't recall it's name.
The Hunt Multi-loader came out in 1959, and was made by both Pachmayr and Safariland.
The Bucheimer / feather touch loader was introduced in 1965 as was the Quick Load.
The Dade Loader came out in 1970.
The modern era probably started in 1973 with HKS loader, about the same time as the Bianchi Speed strip was introduced There have been several others since, but these are still to of the most prolific designs and the oldest still in continuous production.
Bureaucrats being what they are despite being available before then 1970 is about when law enforcement agencies finally started adopting speed loaders, after a shootout in Newhall CA. However it still wasn't anywhere near universal and it took several more years for some departments to adopt them and a few departments never adopted them at all, staying with dump pouches until they adopted semi-auto pistols.