Many flavors of Down Tube shifters
In my mind the classic old 10 speed has shifters on the down tube of the frame but this was handled in many different ways. The most common was to have the shifters mounted on a band that mounts to the frame above a brazed on lug, apparently in the early days there were no lugs and riders had issues with the shifters, under tension from the shift cables, coming loose and sliding down the tube causing paint damage and probably a few missed shifts. Some builders like to put the lug on the top facing part of the down tube (Raleigh super course shown) and others went with the underside (Miyata 912 shown).
|note triangular "lug" below the shifter band|
The route that Peugeot took is a bit different, instead of a lug they use a single shifter boss, on the drive side of the frame, and the (rear) shifter screws directly into the boss. On the non-drive side the band that goes around the down tube has the (front) shifter mounted to the band-like Shimano, Suntour, Campy etc. On the drive side of the band there is a "window" cut in the band so it can go around the boss. Hopefully these pictures will make it seem less confusing that what I just wrote.
|Shifter boss on the drive side only of the down tube|
|Peugeot specific Simplex shifter with window in band to accommodate shifter boss on frame|
|Simplex shifters on a Peugeot UO-8|
For a while in the mid to late 80s you could get this nifty center mounted down tube shifter "cluster" that bolted to the frame -kind of like like a water bottle cage. In the grand scheme of old 10/12 speeds these are kind of rare.
Like the name says these are mounted on the bar ends. This type of shifting is popular on touring bikes where riders want the simplicity and bullet proof qualities of friction but don't want to look down to shift. I like this set up as you can shift with your pinkie while on the bottom of the drops with a bit of practice. These are also popular on Time Trail bikes because you can mount them on areo bars -facing straight out from the bike-and shift without changing your aero profile. Yeah that sounds like a blast....
|Not old or 10 speed but using bar end shifters|
Again the name says it all, they are mounted on the stem. I like these least of the three as they always give me a queasy feeling in the groinal area, must be a guy thing. Anyway the bike companies marketed these in the mid 1970s as safer and easier than the classic down-tube shifters, but I think it was really a cost cutting move as they didn't have to weld an extra bit on the down tube anymore. In general you tend to see these more on an entry level bike than on the higher end "race" bikes.
|on the...um.. stem|
As if those three weren't enough choices you can borrow from early Mt. bike technology and get some Paul's thumbies (or like products) and mount your down tube shifters on the bars! As the name implies you use your thumbs (primarily) to do the shifting.
For trying to teach an old dog new tricks I tip my hat to Retro Shift (http://retroshift.com/) who've come up, in the last year, with a rather unique way to mount friction shifters that combines old with new.