First of all, Herse, Singer et al. were not framebuilders. The French distinguish between a framebuilder (constructeur des cadres) and a "constructeur," which means a builder of bikes. You can't order a Singer frame and fork - you order a bike. Nothing wrong with a framebuilder, like Bianco, but they won't made you a randonneur or camping bike. If you are a racer, you go to a framebuilder, if you are a cyclotouriste, you go to a "constructeur."
The philosophy between Herse and Singer differs. Herse got his start with components, so those always were his hallmark (see the bike in Rivendell Reader No. 26 for a good selection of them). Singer made bikes, and modified components only where he saw it necessary. Stronglight cranks were perfectly fine, so Singer never made his own cranks, but he did offer his own BB with cartridge bearings (which outlasts anything available then). He did make brakes, stems (any custom French builder had to have his own stems), front derailleurs (most then-current designs didn't work on large tooth differences like the common 46/32), seatposts (a bit of frippery here - they had internal clamping). All French "constructeurs" made racks, of course.
Singer is not dissimilar to other constructeurs in the parts he offered - they all made and modified stuff, some more, some less. Most of the complex stuff (machine work for BB spindles, casting, forging) was subcontracted. Levallois-Perret was the metal-working center of Paris, with the Citroën factory and many small custom car shops (as Californians would call them).
I have a wonderful women's bike by Ondet (Lyon). Under the chainguard, it has a custom-made front derailleur (similar to Herse's model), for the half-step gearing. You couldn't do that elegantly with mass-produced parts, so M. Ondet made his own... (The bike will be in Rivendell Reader No. 27 or 28.)
Of course, all of this came about because of randonneuring. You simply cannot build a good randonneur bike by just making a frame and fork and then have the customer worry about the racks and fenders. That is why there are so few good randonneur bikes found on this continent, with the exception of Mariposa, who work in the French way. (Or you can become the constructeur and specify every smallest measurement and detail to the framebuilder, and then hope you (or they) didn't overlook something when the time comes to put it all together. It's a nightmare - I have done it. After the second try, the bike is 95% right!)
For the question of who made the stuff - I hope to have an interview in my newsletter at some point that will elaborate on that. The Japanese Herse book shows how the Herse stems were made from extruded bar stock at the Herse shop. A lot of drilling and filing! I am told odd-length cranks were made that way by Herse as well, but the standard stuff was forged and machined somewhere else. TA made some of the later chainrings, but the early ones with the triangular tooth profile appear to have been made on more primitive equipment, probably in the Herse shop. There is a photo in the Japanese book with what appear to be blank chainrings (no holes cut yet) on the wall.
Jan Heine, Seattle