Based on my observations and my background in genetics and biology, I would agree that the mutant Lemon Zests have different phenotypic shape based on growing in a different environment. I actually have a theory based on what I've observed on my own LZ trees as well as the fruit forming on Franks mutant LZ graft.
If you guys recall, I posted about a similar phenomenon I noticed on my LZ tree. I also had smaller, non kidney bean shaped fruit that were very fat and wide and they were dwarfs compared to a regularly formed Lemon Zest. I just called mine Lemon Zest nubbins and although they do taste different, I do not believe they are true bud mutations. If they are bud mutations, the mutation must be in a gene that is extremely volatile and probably heavily influenced by the environment because I see a similar "mutation" in my trees fruit.
I do not believe it is a mutation but instead, there are environmental factors when growing this variety here in SoCal that cause incomplete pollination of the ovaries. The pollination is sufficient to initiate fruit formation but from my observations, the pollination is insufficient for producing a viable embryo or if an embryo is produced, it is extremely tiny compared to a normal LZ embryo.
We can test this theory by having Frank post pictures of the fruit when it is fully ripe and showing pictures of the inside of the fruit along with pictures of the seed husk and the embryo inside. If my theory is correct, the fully formed "mutant" Lemon Zest will be much shorter than a regular LZ and the seed and embryo will be very thin and have a tiny or almost non existent embryo.
Simon