Hi Wayne,
SunTour top-mount adapters for flat-back downtube shift levers first appeared in '83 catalogs as part of the Trimec drivetrain group. The top-mount adapter could thread directly into a braze-on top-mount boss, or to a top-mount band clamp. In '84 catalogs they were shown as two of four mounting options for Superbe downtube shifters (side-mount direct/clamp, top-mount direct/clamp).
Top-mount Symmetric levers (with self-trimming feature for the front shifter) appeared earlier in '81 catalogs, but I can't remember if standard flat-back downtube shifters would fit.
Veering dangerously off topic, I have a set of Superbe Pro IPC indexed downtube levers on a top-mount band clamp on an '82 Proteus frame. The toptube positioning makes it easier to do double shifts and trim the fder after a single rear shift. Any standard flat-back non-indexed on-topic shifter would mount just as well.
The top-mount adapters and band clamps were available as replacement parts way back when, but I don't know where you could easily find some today. I did a quick search of my parts boxes but couldn't find any extra top-mount adapters. I'll noodle around a little more when I get the chance. They must turn up on garage-sale bikes. Google may help?
Don't know if Shimano ever offered something similar. If your Shimano top-mount levers are similarly standard flat-mount dt levers, you might be able to graft the SunTour levers to the Shimano top-mount adapter.
I once had a Klein Stage frame ('83?) with a Klein homebrew top-mount adapter---the frame shipped with plastic Modolo shifters. First day the shift lever cracked in half, then a few days later the adapter's rivets failed and the adapter pulled loose from the frame. Klein sent a side-mount dt conversion kit, which didn't look as cool but was much more reliable. With the braze-on boss on your frame, a homebrew adapter might be possible...
Cheers and good luck,
PB <sig follows below...>
On Wed, 7 Aug 2002 10:09:39 -0400, "Bingham, Wayne R."
<WBINGHAM@imf.org> wrote:
[snipped]
>was there ever a device that mounted to the top-mount
>braze-on (essentially a small block with a threaded hole in the top)
>that would accept conventional friction shift levers? Anyone ever seen
>such a device? If they exist, does anyone know where I might find one?
Paul C. Brodek
Hillsdale, N.J. U.S.A.
E-mail: pcb@skyweb.net