Appears to be a lovely car. My father was a Cadillac dealer back then and I was active in the business; dont ever remember putting 68 side marker lights on a 67 Eldo, but we were a relatively small town dealer. But ....... so what, theyre there and they look fine, just not original, most people would never know. I dont even remember if they were on the 67s or not, and wouldnt care. If youre thinking of buying it keep in mind the 67s were the first year and were basically custom built oddballs. Many of the front end components were one (or 2) year items only and replacements for some wear items are most difficult to find. I dont follow Eldos though and there could well be someone out there that specializes in parts for the early ones, if there is (and you buy the car) you should find him/them.
Just very recently a 68 Eldo owner, Kevin Penberg, was posting at the Buy/Sell site looking for front end parts for his car. You might go to the Buy/Sell site and scroll down to "68 Eldo rotors, bearings, and ball joints", first posted on 7/19 and read the threads there about parts.