When I worked at Austin-Healey West, San Francisco circa 1978-'79, we had tri-carbs (>Mar '61*) come in that were both side & center-shift. In fact, I'd seen all side-shifters up to some point, and questioned the authenticity of the first center-shifted one (>Nov '61*) before Ray edgu-makay-ted me on the mid-series production change. Reverted back to twin carbs with the "Convertible" model (>Jan '62*).
That left a roughly two (2+) months of overlap with three (3) carbs and center-shift xnsm
The side-shifted cranked up lever was introduced in >Aug '60*, though I'd seen tri-carbs with the straight lever fitted during my tenure there too (whether by personal preference, xmsn swaps or limited factory parts availability I do not know).
If there is One Thing I know for sure about Healeys, is that you CAN'T BE SURE about anything!
* ALL references to dates credited to Anders Clausager, Original Austin-Healey