Q.planted meyer lemon tree with extra branch at the bottom of main shoot
In January 2013, I planted a Meyer lemon tree that had an extra branch at the bottom of the main shoot. From that extra shoot produced 7 lemons the size of an orange. This year there is only one lemon on this shoot. Should I cut this branch off? It is now about seven ft tall with new growth. The tree is 6 ft tall with new growth coming out of the top branches.

In all probability, that extra branch is being produced by the root stock onto which your Meyer lemon was grafted. The usual advice is to prune off any branches sent up from the root stock, but I'm wondering, did you try the fruit from the rogue branch? If you liked it, you could let it grow instead of pruning it away. However, generally you would want to prune it off, to let the tree concentrate its energy into producing the fruit you want. This article has more info on grafted fruit. https://www.gardeningknowhow.com/ornamental/trees/tgen/can-grafted-trees-revert-to-their-root-stock.htm