The mechanical secondary modification for the 3 barrel and the 3310, consisted of a tube, with a Y at the end. It pressed into the accelerator pump nozzle and extended across, to the back of the air horn. You drilled two holes, one on each side of the air cleaner stud boss, for the Y to feed the secondary's and you had to used the 50cc pump diaphragm. There were kits listed for them and I have seen quite a few 3310's, with those holes in the air horns, over the years but, the Y adapter missing.
When they came on the market, the 3 barrels where fairly cheap (at least in So Cal) and the 3310 was very affordable. I bought 2 of the 3310's for a MT, cross ram manifold, in '68 or '69.
Frank