http://www.moormodels.co.uk/acatalog/Black_Horse.html
john.
here is a link to a picture they still sell them.
its not a trainer but will use it as an intermediate after my wot4 by chriss foss.
hope this answers the questions you raised.
have found another link on a different sit that lists 2 different measurements one as 100mm and another from same model 7 years previous at 125mm so will be interested to see how your formula works out on mine.
many thanks...
have also now discouvered that the installed 138 df reciever has also died ,, twitches when switched on but no movement of the survo's. ah well new reciever too :-/
keith