SpeedofLite 

Announcement of Dual Position Brake Levers on most drop bar-equipped Schwinn bike models (Jul 1969) followed by a Letter to the Editor concerning same (Sep 1969)
I just find it historically interesting and funny that controversy has long seemed to be associated with these levers.
The Sheldon Brown glossary term Extension Levers provides additional insight and amusement, at least to me anyway.

non-fixie 

"To popularize adult cycling" is what looks alien to me, as here in Europe "adult cycling" was never an issue.

The dual action levers did appear here, more or less as a successor to the "guidonnet" levers, and only on touring-oriented bikes. And not just on the cheap ones either:

Salamandrine 

That's funny to read now. To some degree "safety levers" were always unfairly maligned. The usual enthusiast view was that they were actually hazardous since they didn't stop as well as the main levers, and that they gave people a false since of security. May have been a little truth to that, but it was likely snobbery had a lot to do with it. They worked fine for casual riders, and I can't recall anyone ever crashing because of them. BITD we did however advise people to use the drops for serious braking.

OTOH I have seen people crash when descending on rough roads on the hoods on modern bikes - more than once. So at least for now my view is that brifters are the real suicide levers.

It's my understanding that they were invented by Dia Compe, and shared with Weinmann in exchange for licensing rights for the Weinmann centerpull caliper brakes.

That sentiment is all too familiar to me. Bikes were considered kid's toys from WW2 until the bike boom, here in the US. My parents generation sees them that way for sure to this day. Bike racing was actually extremely popular in the US in the 1930s, but only in certain pockets like NYC. I remember the football coach at my old high school describing it to me. He said it was as big or bigger than baseball. Even so, from what I've read general cycling as an adult form of transportation and sport had been in a slow decline since the great bike craze of the turn of the century.

non-fixie 

I think it's all Henry Ford's fault. He made cheap cars available in the US in the early 20th century. In Europe cars were luxury items until the late fifties. And by then the bicycle had already secured its place, both in sports and in our daily life.

In France, the newspaper boys raced, in Holland the butcher boys:

repechage

We referred to them as "comfort levers" to our customers. Advising that the best braking power and modulation was from the drops.
The Style of the day was to own a "ten speed", showing the basic attributes of a racing bike.
The dual position levers allowed one to not lean so far forward and brake from the tops, popular.
So much so, that many Peugeot UO8's were fitted with them, a nice upcharge to the original purchase price. A local mfg made the replacement pins that accepted the Weinmann or Dia-Compe levers, the pin was held in place by a circlip on the outside on the Mafac lever replacing the original pin. The new pin was a slide-in fit, not a press-in as original.

Changing Weinmann or Dia-Compe levers were more work, the lever assembly had to be taken almost completely apart.

More money was to be made replacing downtube shift levers for Suntour power ratchet stem shifters.

bikemig 

Dual position brake levers get a lot of flack here ("turkey" levers, "suicide" levers). I'm with @Salamandrine, they were unfairly maligned and they did sometime end up on nice bike like @non-fixie points out. I picked up a 70s Sekine SHS 271 with dual position levers. I love the bike and the dual position levers do a nice job of modulating your speed. Other than the saddle and consumables, it's pretty much all original.

dddd

Bikemig's Shimano version of the 2-position lever was quite superior to Dia-Compe's add-on lever for two reasons.

First, the aux-lever version didn't lose any lever travel to the addition of the side lever.

Second, the pivoting of the side lever acted across two more widely-spaced bushings inside of the lever body, so the lever pivoted more accurately under load and was not susceptible to loosening or falling off.

The side lever on Shimano's version is actually the main lever internally, acting directly on the cable anchor catch, with the "normal" lever then actuating that mechanism inside of the lever body.
The large-head screw opposite the side lever actually rotates as the side lever and pivot shaft rotate together, thus I had to cut a large relief hole in the hoods that I fitted to them so that the lever movement wasn't slowed by contact between the screw head and the rubber hood. The complete lever assembly had to come apart in order to fit the hoods, perhaps why rubber hoods were never specified for use with these Shimano levers.
Anyway, the braking actuation and lever travel of Shimano's side/aux levers is superb!

dedhed

I have a bit different Shimano set up (DEL-80) on a old Koga -Miyata I need to get rid of.
They connect to a hole in the lever and have plastic grips.