In Solex World, all Solexes sold in the US after 1974 had to have brake lights. The model 4600 DOT (4600 V2 aka The Horse, and 4600 V3) was created for this purpose, equipped with all the safety features required by the Federal Safety Standards, including brake lights, electric horn, reflectors, ignition stop switch, sealed-beam headlight, etc. Some States required brake lights on Solexes as early as 1964. When the S 3300s, S 3800s, 4600 V1s and 5000s were sold in these States before 1975, a switch (by MALY) was installed inside the left frame rail, attached by a pullchain and rod (old spoke) to the rear brake cable. The switch was normally closed, held open by the pullchain. When the rear brake was activated, the brake cable (and pullchain) moved forward, closing the switch and allowing power to the brake light bulb or brake light filament. I have some photos posted on this page:
The whole setup came as a kit from MALY, and I've seen the kits for sale on ebay.fr, and installed on other mopeds and motorcycles.
Here is a list of all of the safety equipment required on a moped in New York State:
Technically, in NYS, if you don't have this safety equipment, you are not allowed to register the ped or ride it on the streets.