Re: 1973 Mercury 1500 I-6 with 65' or 66' Ignition Components?
Glad you found some good deals on the parts, that was a much better price on the coil!
On the switchbox, either 4-cyl or 6-cyl box will make sparks, doesn't matter if used on a Four or Six. But if you were to use a 4-cyl box on a Six, or a 6-cyl box on a Four, the OEM Merc tach won't read correctly.
Unless you've got a Mercury old-style tach, you'd connect the tach sensing wire to the rectifier. This will be a brown wire coming from the motor's internal wiring harness.
Note that with your strange setup, you might find the brown wire connected to one of the twin-coils, as this would work with a car tach set on 6-cylinders.
If you have the old Merc Quicksilver tach that goes to the box, you'll need a new tach. Unless, of course, through luck-of-the-draw, the switchbox you get is for a 6-cyl motor. Usually, a 4-cyl switchbox will have a big "4" sticker on it, and a "6" sticker for 6-cyl motors.
The eBay vendor is technically right in his description, it will "fit" and work on the models he listed, with exception of the tach. Not an issue for most folks since the OEM Merc tachs are getting quite rare or have died by now.
Note that these switchboxes all have the "2986" prefix, but the "A#" tells you exactly what motor it was intended for. For example, a 1975 850 used a 2986A22 box, while a '75 1500 used a 2986A25 box.
'70 800: 2986A8; '70 1350: 2986A4. All these numbers have been superseded by newer ones, but they still retain the core "2986" number.
So, any "2986" switchbox will probably work, with the above caveats.
Here's a shocker: the 1500 switchbox is NLA from Mercury but the last listed price was over $395! Aftermarket #'s for this box are Sierra 18-5779 or CDI Electronics 114-2986.
Here's a 6-cyl box for $60 or best offer:
Used Mercury 332 2986A21 Switch Box | eBay
A little scruffy, but guaranteed to work.
There's a way to bench-test all these parts before you install them, once you've got 'em all gathered up, we can discuss.
BTW, a new rotor shaft (when last available) was over $462 and a distributor cap is almost $141 so I reckon that wasn't too bad a deal on the complete distributor, eh?
HTH........ed