Have you measured you worn-out cone to verify it has a weird non-Campagnolo diameter or threading ?? Place it on a campagnolo axle and see if it matches a Campagnolo rear cone ... if it does ...
In this era Avocet made 3 levels of hubs : Avocet Model I, II, and III.
Model III was always the best stuff - super light - with sealed bearings.
If you have cones on your hubs you probably have Model-I or Model-II parts.
In most cases in the 1970's, there was "Italian" equipment (almost all of it compatible with Campagnolo, e.g. Gipiemme, Ofmega, OMAS, Miche, Zeus), and "Asian" equipment (almost all of it compatible with Sunshine / Sanshin, and later, Shimano which conformed to Sunshine / Sanshin specs.) Even today, manufacturers are still building these two flavors of parts as spares :
M10 x 26 tpi - Campagnolo Compatible axles & cones. M10 x 1 mm - Shimano Compatible axles & cones.
Genuine Campagnolo M10 x 26 tpi cones :
http://www.loosescrews.com/
I think that the American Company Wheels Mfg makes a compatible part CAN SOMEONE PLEASE VERIFY THIS (Joytech 085) :
http://wheelsmfg.com/
A third option is to replace your M10 x 26 tpi axle set with an M10 x 1mm axle set from Taiwan, there are many generic versions of these axle sets for less than $10 that probably conform to campagnolo specifications (the main thing you need is a cone that's not too wide in diameter, placing the bearing surfaces at an angle that is reasonably close to OEM campagnolo, since axles flex and the cups have a circular radius to accomodate flexion in the axles, so the angle of the bearing cone can be different.)
- Don Gillies
San Diego, cA, USA