To my knowledge no one has even resolved if doing a FASx on mescaline base produces mescaline fumarate, mescaline bifumarate, or some mixture thereof.
Which, aside from dose calculations, is a little academic because no one has measured the solubilities of either pure compound.
I suspect that a carefully conducted FASx will produce mostly mescaline bifumarate.
When I made true DMT fumarate by titration what I got was a maddening goo that only got thicker and thicker, it in no way resembled the product widely reported by FASA production. I found another who produced DMT fumarate by titration and their experience was the same.
It annoyed me so greatly I just dissolved it in water, drank it, and shattered the universe. Take that, goo!
I then produced DMT bifumarate by titration and I got a lovely crystalline product that was very easy to work with and not noticeably hygroscopic, I desiccated it out of habit rather than necessity. Making pills was a pleasure.
Producing mescaline bifumarate and testing its solubilities and recrystallization behavior is on my long list of things to do
[I anticipate that its recrystallization behavior will be second only to the sulfate]
I doubt I'll work with true mescaline fumarate, as I expect it to be a sticky mess.