For the purpose of making an ATR which actually mounts (and the latest discussion seems to have been sparked by the result of attempting to prepare an unARCed copy of the SDX toolkit disk with an ATR tool which couldn't manage to write the header properly), only the first seven bytes of the header are required, and these are common to both variations.
It is the 1st 6 bytes that are common. That's probably what you meant to say. In Nick's a word is used at offset 6 and 7 for extending the amount of paragraphs . In Steve's it is a byte at offset 6. Offset 7 is the first part of a dword for his CRC. Granted it really would be unlikely that a full extra word would need to be added to the image size information and single byte is fine. The other areas that are common are Nick's spares at the end (offset 11 - 15), except for the last byte (offset 15) which steve wanted to be for flags, even though Nick had already defined that at offset 8.
Since the spares do not yet have an agreed upon funtion, in fact no one has yet defined them, they are not "safe" to use for an image that needs to be compatible. This leaves only the the 1st 6 bytes as safe and useful right now.
Edited by fujidude, Tue Feb 17, 2015 7:15 PM.