Back in the 50s and 60s, the running joke in the industry went something like this:
Salesman- "Look boss, our model of the Onion Specific 460 Grobnick we announced earlier this year has proven very popular with modelers over the last six months."
CEO- "That's great...I guess we'll have to actually design and manufacture one now..."
In Athearn's case, they did in fact make them but, their mechanism proved so unreliable, that they went though a complete redesign that took almost two years to complete, delaying this model until 1961...
Word is that several of these early Hi-F drives did escape the factory, though no one has ever seen one. In the world of HO, it would truly be worthy of the words "rare" and "highly prized".