Now that intrigues me again - 434.075 is within 70cms but I'm assuming the frequency is dedicated for this sort of thing, unattended airborne operation. I guess because 70cms is a shared band whereas 2m isn't that you can do this without any problem. Do you have to have a callsign attached to the RTTY or is that not needed? I ask that because I noticed you had a call on the beacons from Apex I as well as a beacon path
434.075 MHz is not a dedicated frequency, but it is extremely rarely used. It's designated as channel 41 of the LPD433 frequencies, but very few devices seem to use it. Hence the reason balloonists began to use it, and now the distributed listener network uses it as well.
The callsign system on Apex I was in use primarily because the system used APRS. This meant we used existing software and code, which were designed for amateur packet use (AGWPE, APRSPoint, ax25d, beacon, libax25). Of course, these required a callsign. Apart from making the system compatible with existing software, the callsign served no other purpose.
The listening daemon on the flight computer was locked down to respond to packets from only one callsign, but any SSID. This gave us some degree of security whilst allowing multiple base stations control over the onboard avionics.
On Apex II, a callsign will also be used. This is required as part of the message transmission protocol (link). The distributed listener network feeds to the internet, so when watching the internet feed, the callsign can be used to identify a particular payload if there happen to be multiple ones.