AMIBIOS Rev-X2-03 is a version of AMIBIOS, assumed to be made by Access Methods (which then would sell AMIBIOS to American Megatrends). This specific subcore is assumed to have been released in 1985 or early-1986.
It can be seen on an unknown IBM XT clone motherboard demonstrated by the YouTube channel Dfgrt924. This is believed to be the earliest known subcore of AMIBIOS with proof to confirm it's existence.
POST screen
The POST screen is different from all other known versions of AMIBIOS. It also has two memory counters (one for base system memory and other for expanded memory that could be additionally installed), instead of only one as seen on other versions.
This version also seems to lack a proper BIOS string, while AMIBIOS 2.2x already had a string, indicating the string was introduced with 2.2x or a slightly earlier version. It says "Rev-X2-03" where the string would go with future versions.
The BIOS also lacks a setup launch prompt, which may indicate the BIOS was compiled before 1987 as that's around when a setup utility was introduced.
Boot error messages
There are two boot errors seen in the demonstration, and they are slightly different compared to the error messages seen on other versions of AMIBIOS.
First error:
Drive not ready
Insert diskette in A:
Press any key .....
Second error:
Boot failure
Insert diskette in A:
Press any key .....