Is every 50-bit number a valid WSPR transmission?

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Is every 50-bit number a valid WSPR transmission?

As I understand it, WSPR encodes callsign, grid locator, and power into exactly 50 bits (28 bits callsign, 15 bits locator, 7 bits power), prior to channel coding.

Will every possible 50-bit number result in a valid WSPR decode? In other words, if someone transmitted using 50 random source bits, would that be guaranteed to show up on WSPRnet with some random callsign, location, and power? Or are there "holes" in the space of 2^50 possibilities?

I ask because I am seeing telemetry applications jumping through all kinds of weird hoops to encode data in callsign, locator, and power bits which are then passed through the standard WSPR source-coding. Wouldn't it be a lot easier to just bypass all of that and encode data optimally into 50 bits, to be passed directly into the channel coding? (I realize part of the difficulty lies in avoiding any real callsigns not your own.)

A related question is how this fits in with ID requirements. If a balloon is sending WSPR with a bogus callsign, how do they identify? Are they sending CW IDs between transmissions, or is it deemed acceptable to have a "normal" WSPR transmission preceding each telemetry-only transmission?

I tried looking at the VE3KCL ( and qrp-labs) sites but it is clear as mud to me. I'm an engineer so I need a technical description, please ;)