It's sad that this is probably going to become canon, now, Raymon Chen being as influential as xe is, when what actually happened was that in 1998 H. Peter Anvin saw UD2 in Intel's doco, and that there was a second opcode that was just described by Intel as merely undefined, so gave it the mnemonic UD1 in the 0.98p3-hpa version of NASM because 'calling it UD1 seemed to make sense'.

* https://groups.google.com/g/comp.lang.asm.x86/c/ErG5TJEDjiw/...

* https://github.com/netwide-assembler/nasm/blob/nasm-0.98.x/C...