According to Keith Williams :
In article ,
says...

There have been non-Robertson square drive screws and drivers available
for quite some time. They're so interoperable it's seldom possible
to tell them apart so we call them _all_ Robertson - but many of them
aren't really. Patent-wise at least.

Interesting. What's the difference? I thought the patents ran
out, which is why we're seeing them (for at least 30 years) in the
States.

As far as I know, the patents are still in effect - reengineering or something
silly like that.

Robertson's primary patent claims are a very precise taper on the tip,
so as to be better at gripping and releasing the screw. Square drive
normally has no taper.

Years ago the taper probably mattered more, because of manufacturing
tolerance meaning that a taper was _necessary_ to get a good interference
fit. These days manufacturing is more accurate, so a no-taper tip will
grip too. Particularly in screws made to Robertson specs.

I recently bought some fairly high-grade square drive bits (these were
part of a very large surplus shipment sent up from the states and resold
here). They grip "real" domestic robertson screws real well. In fact, too
well at times, and you have to give a bit of a yank to get them to disengage.

[Unlike the umpty-ump versions of Phillips which all behave noticably
differently.]

There are some that look like Phillips but are most certainly not.
HP used a screw that looked like a Phillips, but wasn't (IIRC it
had straight sides rather than curved). There were four radials
stamped in the head indicating the special head. I don't remember
the name of the head.

A recent bit set I bought had three distinct "cross heads" with slightly
different geometries (and 3 sizes of each). One geometry was classic
phillips. One, I believe, Pozidrive. Don't know what the other was
called. Any, of a given size, would work more-or-less on a screw.
But if you wanted a good tight grip to minimize cam-out (especially
when you were going to drive a lot), you have to experiment with all three
types (at one or two sizes each) to see which one did best.

At least with square/Robertson they're interchangeable at a given size,
and if you pick the wrong size, it's real obvious (won't fit, or will
be _very_ sloppy).

Strictly speaking, McFeeley's (for example) sells square drive in the US,
not Robertson.

They may sell them, but aren't they still the "Robertson head"?

No. "Square drive" - interoperable with Robertson, but still not
Robertson. The name is trademarked too.

BTW, I was referring to the 1.6Gal flush and "outlaw" toilets, not
the screwdrivers.

Either way ;-)

The "outlaw toilets" are outlawed in the US.

I'm aware of that.

Actually their sale
is outlawed, hence the trip north (less than 50mi.) if I need to
replace one.

If you need one, you'd probably have to hurry. Most toilets
sold here these days are low flow too.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.