The "triple extension" was an after-market item (later offered on some japanese derailleurs as stock) that I saw advertised in bicycling magazine in the 1970's. Sighting from the rare dropouts towards the crankset, I believe it looked like this :
tab || \/
\ O <= cage screw goes through this hole only. | | +===+
The inner side is a tab that fits through the inner hole at the back of your derailleur cage. The outer side has a hole, and a very small flat screw was used to affix this "drop-cage" to the regular cage. It added maybe 5-10 mm to the vertical length of a derailleur cage.
Did it work o.k. ?? Does anyone still have these ?? It should be pretty easy to make this out of a soft piece of steel.
- Don Gillies
San Diego, CA