<p>After the brick, the self-test still printed garbage in four of its six sliding-ASCII diagnostic rows, because those rows render out of the two fonts my blob is sitting on.</p>
<p>Turns out the stock ROM already ships a DIP-selected quality lock at 0x0ACB that forces every font class to one of the two fonts I did not touch. Making it unconditional is one byte, <code>0x0ACE: JR -> NOP</code>.</p>
<p>Forty years later and whoever wrote this thing had already left me the exact lever I needed. v5i then puts the visual variety back a different way, by rewriting the class-to-parameter-block pointer table at 0x0AFF so every class resolves to an intact font base while the metrics still differ.</p>