Long story short.
I saw a post about bank swapping the basic and a game on the 1200xl. This made me think about my xegs that I have in my repair pile. That has Missile Command internally installed on the system. So through a series of experimentation, revelations, leading me to eventually pulling out the hex editor. I determined what is partially stated in some of the references I have linked below. The C101687 rom is a concatenated file containing in this order: Missile Command 8KB, RevC Basic 8KB and Rev4 OS 16KB. The xegs C101687 ic is Jedec compatible, meaning I can use a 27c256 eprom (32KB / 32,768 bytes). The concatenated file must be that size, not a byte less or more. So the goal is to change the game. Doing limited testing, I tried multiple different extensions, xex .rom etc. The .rom extension was the only one that gave me limited success. Limited, because not every 8KB game I eventually tested worked. I used a tms27c256 eprom, Hxd as my hex editor, which also works for file comparing and concatenating the multiple files on hand. Some days Command Prompt is just not fun to use. lol.
End result is I was able to concatenate my own working file containing Missile Command, RevC Basic and Rev4 OS. Later on, I did the same with Qix, RevC Basic and Rev4 OS. Below are all the references I used. In addition to my own log. That contains every assumption and conclusion made during my experimenting. Whether it was right or wrong, I left it it in my log. Hopefully others can learn from it. Picture below references.
My Mod and repair blog including this : https://www.gatesgam...ruary-2018.html
Rom sources http://www.atarimania.com/
Basic info http://www.digitpres...8ataribasic.htm
OS/Basic rom source http://atari.vjetnam....php?frame=roms
IC identification https://www.atarimax....org/achip.html
Post that triggered my experimentation http://atariage.com/...=1#entry1875799