RonB wrote:

Well, you could get all tooled up with a hollow chisel mortiser,
or all burly with a mallet and a mortising chisel,
but if you are more into getting the job done fisrly fast and
not buying fancy tools, a good sized router with a carbide bit
and a jig should get the job done

This router approach is kinda what I had in mind when I asked my
question. I suspected it was a rail-type of fence. You probably
wouldn't have to go all the way through...just a couple of inches
to allow the boards to be inserted and fastened.

I should have thought of that. No need to go all the way through the
posts, except that it means the slots are automatically aligned
properly.

I have a floor-standing drill press with a cross-slide vice, I also have
a relatively large router - but no router table. It would be more of a
hassle to set the router up to do these posts vs the drill press.

I suppose I could use a router bit on the drill press.

There are also drills that cut side-ways (don't know what they're called
exactly). If these are cedar posts, I suppose it's soft enough to try
one of those drills instead of a 1" diam x 2" deep router bit.

Pay attention to drainage or you'll have rotted boards in
short order.

I've had some good experience with using PL-Premium adhesive on exposed,
outdoor wood. Seems to hold up well under intermittent water exposure,
and winter/summer temperature swings. I'm thinking that using liberal
amounts of that glue to hold the boards in the slots (and sealing up the
slots in the process) might be the way to go. Yes?