The A ROM (8F46) and C ROM (500C) contained here were modified by Frank to fix various issues with the stock Model III and M4 ROMs. The stock B ROM (407C) was taken from MAME and included so that a complete set of ROMs could be re-created.

Ira did a great write up on the C ROM here: http://www.trs-80.com/wordpress/zaps-patches-pokes-tips/model-iii-rom-explained-part-4-durda/