Bush lemons, eureka, Villa Franca, Lisbon. They’re the ones I have. I’m sure there’s many more. You can tell a Meyer lemon by the smooth shiny skin and large amount of juice compared to the old fashioned varieties.
When there’s a glut of lemons, I make up many bottles of lemon cordial, just for jam making. It might go dark over the years but it still works to set the jam.
I’m not into sugar, but in a prepper’s world it makes all that wheat turn into tasty pancakes.
Apples are high in pectin and will set jam too, especially the green apples like Granny Smiths. They’re also good to extend fruit quantity without messing with the flavour too much. Eg, raspberry and apple jam. I thought I’d cheat once and just dump a bottle of bought apple juice in the jam with the thought it would be more of a jelly. It didn’t work because the juice was reconstituted. They must take out the pectin in the processing (probably to make jamsetter).