You have a board in your DAC that was made by @jkrichards. Jeff is a well known modder of the MK1 and MK2 and is very active on the forum. He hangs out over here:
Your MK1 may also have a transformer mod and a few other circuit changes. You’ll need to pop the top and look for some component additions.
The external power supply is most likely a Zero Zero. Does it look like this?: