The one that is labeled as 128K, though, seems to use the Mac 512k memory map - 0x7fd00 for the sound buffer (see the instruction at offset 0x1f8) - rather than the 128K memory map - 0x1fd00 for the sound buffer. (Similarly, at offset 0x102, the ROM is using 0x7a700 for the video buffer instead of 0x1a700.) Is it really a 512k ROM and not the 128k? Or is it designed to work on both? I've seen the same ROM checksum labeled in a couple different ways. I was expecting it to match the memory map info for the 128k that is visible here: http://www.osdata.com/system/physical/memmap.htm#MacPlus
Any suggestions to make sure I'm looking at the right resources are greatly appreciated.
The sound and video buffers aren't really part of the machine's physical memory map, but are software constructs used by the ROM and system software. Those buffers are relative to the top of RAM, which is why in the chart you linked, it is in different locations even on the Plus with different amounts of physical RAM. The ROM isn't hard coding the location, rather it has determined the size of physical memory and placed the buffer at the appropriate location.