Hello John, you can find the list of country codes in the technical manual, above left, just search "dealer and country codes" when you're on the main menu page. If you haven't already found it, there is also an explanation of data cards, search, strangely enough "data cards"
Finding a list of dealer codes from the period seems to be difficult, and even if you do trace the dealer, if they still exist, in this age of data protection, they may be reluctant to provide previous owner information
As i understand it your order number deciphers as 5 - denotes the date of 65, 705 is the country/region code - 703 - 706 USA, Puerto Rico, Guam, USVI, American Samoa etc. (703 was used only briefly) and the last four digits is the actual dealer
If you also search" previous owners", or "dealer codes", using the search function above, you'll find many similar posts as yours, so you may get some more tips there
Good luck