Looked at a English Best pigeon gun today and someone had installed what looked like a Fabbri single trigger.
Any ideas on who could have done triggers like this?

Best guess would be Ken Eversull.

Ditto on the VERY Talented Mr. KE

I’d third that. Ken is probably the best Fabbri guy in the Western Hemisphere.

Does some external feature indicate a Fabbri trigger? Does Ebersol claim to install such triggers?

Mr M.
It is Ken Eversull, and he has the trigger components, and the skill set to apply them where possible (sidelock gun configuration) The Fabbri trigger is identifiable INSIDE the gun, not by any visible external appearance of the blade or any markings.
The craftsman is HIGHLY SKILLED and priced higher than many surgeons, but then again there are a lot of Surgeons relative to the total population, and .....only one of him.
Be prepared for some degree of sticker shock......

I could be mistaken, but I think Ken apprenticed or worked at Fabbri for some period of time.

I believe Ken was/is stateside Fabbri approved service/repair guy. Also heard through the grapevine that he refined the Fabbri ST early on to what it is today - better. I may not have this correct, but the guy is GOOD.

Buzz, for a number of years , some time ago, KE spent a good bit of time in Bresica at the Fabbri facility honing his craft.....and keeping abreast of the innovative minds of Ivo and son.
He is a talented and gifted gunsmith.....

Not just GOOD...probably "THE BEST"