So I know it’s possible to run first gen CPU’s on select B550 boards without issue, namely Asus and supposedly MSI. At some point Asrock had it on their B550 Phantom 4 but it seems to have changed since 1.10, as indicated by the 1700 I have not posting on it.
I have a CH341A on the way to fix a bad bios flash on a Asus B450 Tuf but I figured since I essentially have a failsafe device, I could get a experimental and see if support could somehow be added back.
In theory it should be possible, huh? Especially if it once worked on the board. The whys are… Well, if I got this board on hand but only a 1700, I’d like to just hang onto it until I upgrade the CPU later and just flash a stock bios on it.
So I got the CH341A yesterday and was able to repair the ASUS B450 Tuf using the wealth of info by @Lost_N_BIOS _bios as it seems that AFUDOS decided to write over all the UUID and MAC info when it was trying to downgrade. Luckily I could verify the MAC and UUID through Linux’s dmesg logs and just filled it in from there using FD44. And boom, board was resurrected. 
Going a step farther, I then downgraded the Asrock B550 Phantom 4 to bios 1.0, which did (kinda) allow support for a first gen Ryzen. Just couldn’t save any options in the bios, as I had read before anyway. I just didn’t realize that when people said that, it’d prevent POST till CMOS clear. However, downgrading DOES allow it to POST with a 1700.
However, downgrading DOES allow it to work. I went and upgraded within bios to 1.70 just to see if maybe they silently added in full support, but no. It would no longer POST with the 1700.
So what/where is it in the bios that allows this and is there a way to maybe add in support in newer bios?