Hi folks ... so I got my Speedport 504v to run with the initial installation images on the old wiki page.
The web UI gives this version: OpenWrt Chaos Calmer r46134 / LuCI Master (git-15.176.44391-0ef5f3c)
So, I thought, let's get this up to the most recent version and I downloaded lede-17.01.4-lantiq-xway-ARV8539PW22-squashfs-sysupgrade.bin
Verified the Hashes are correct, but after uploading it, I can see a kernel panic on the console.
Hmm ... well, I did flash the older image via serial console, but I'm unsure about the right procedure to flash a sys upgrade image. I was getting the impression that that was not a complete replacement. On the other hand I've read that the factory.bin images are just special in that they are made to be accepted by whatever the original vendor's FW update utility requires. It seems to make sense, too, considering I've read the original boot loader requires signed / encrypted FW. Could be that's why there's no factory.bin.
If that's the case and I can basically just flash the new sys upgrade image with the same u-boot / xmodem steps that I used for the other, then I might give that a shot.
Sysupgrade.bin's lack the OEM header that some devices need to see when flashing to OpenWrt from OEM firmware... that's the only difference between a system.img and sysupgrade.bin.
Actually no ... I noticed I could get to busy box on the serial console and stuff was actually running.
Turned out, the box booted fine afterwards.
I'm a happy camper, thanks for the suggestion.