+1 AD
You may want to research the AD a little bit, too. It just might be an old mid-70's Ultima road racing frame. These were purple 531 frames with gold pin-striping around the lugs. AD's top of the line, along with the favorite collectible Vent Noir sport bike. My 1976 Ultima has braze-on downtube shifters and braze-ons for water-bottle cage only (no top cable braze-ons). If in good condition, these are very nice frames. Ultimas were originally fitted with all Campy SR, so maybe, just maybe, someone earlier on has swapped out the campy for the Shimano parts and now the bike is for sale as just an old $75 bike by an unknowing seller. Of course, it may be a lower end AD model, but with Reynolds 531, it is probably a nice bike if the frame is straight and true. Be careful, though. The AD might not easily build up into what you want. My Ultima is fast but very sensitive, certainly not a long wheelbase, fat tire cruiser in any sense, but great at what it does. Sorry I can't post a picture - not logged on to my own computer - but my Ultima has been posted on here long ago and may show up if you do a search.
Best Wishes!