Hartzell goes after Top Prop STCs to try to get certain propellers out of the field. Basically they want to get their older, highly AD’d props out and replace them with propellers that have demonstrated increased reliability. There’s a certain level of interest in displacing McCauley, but not quite as much of one.
I suspect the 340 is of minimal interest to them. I’m not sure of the AD status on factory 340 props, but since many 340s have RAM conversions that come with Hartzell props (at least the Q-tip equipped models), they probably view it as extra effort to STC a prop on that would have relatively few sales and few benefits. As an example, they’re pushing the Top Prop conversion on older 310s that have props with highly restrictive ADs on them.
I asked them if they were going to go after an option that would displace the McCauley props on my Colemill conversion, and they said no – I should contact Mike Jones (who owns the Colemill STCs) to see if they would go after such an effort. Although field approvals have been done on singles for propeller swaps, I can’t find anyone who’s aware of that happening on a twin (including Hartzell).