Ok here goes, as nearly every year has something distinctive item on the exterior or on the dash or front suspension that will determine the year, IF it's still original and folks haven't swapped other year parts on and off of it over the last 50+ years. Here's he basic and easy things to look for.
66 and 67 had NO side marker lights or reflectors. 66' has a door keyhole just above the trim line, 67' has the door key hole almost touching the door handle(because in 67' the Sport package was added and the belt line time would go where the key hole was on the 66'). Those two years had their own tailgate that had a ledge at the top, rather that a "hump" that ran the length of the top. 66 and 67 shared the same dash, with long dash speaker slots and the ashtray was close to the steering wheel, whereas the slots were narrow and the ashtray was near the glove box on 68-77. All 66' dashes were painted bronco gray poly, the 67' dash was parchment. 66 and 67 shared the same dash pad, narrow all the way across whereas the rest of the years have the hump protruding out towards the driver. The 66 and 67 have about two dozen other items that differ from the other years. The 66 has about 2 dozen times that differ from the 67', and the 67' have about a dozen items that differ from the 66 and the other years. I'm not going to list all of them for the 66/67 and the 66 and the 67' lol.
67-77 were the only years to offer the Sports package(lots of chrome trim)
66-68 have the same grill(except for first three months 66 grill) that use the smaller, clear turning lens as opposed to the larger amber signals. 66-68 doors have removable window frames, so if the doors are open, you'll see bolts along the door for them.
66-68 were the only years with 289 fender badge.
66-early 69 had vacuum wipers, so their was a hood loop, and the passenger windshield wiper was much further to the passenger side. Late 69-77 no hood loop, and the passenger wiper was almost in the center of the windshield.
68' and 69 have side reflectors, no bulbs just reflectors. So if you can see the grill and the side, one can distinguish between the 68' and 69'.
66-70 have solid headlight rings with no notches, 71-77 have four notches or slots every 90 degrees, top and bottom, left and right.
69-71 were the only three years to have the 302 fender badge if v8 equipped.
70-77 have horizontal side marker lights, the rear marker light on the 77' was vertical due to the fuel doors.
72'-76' are the trickiest years to determine from just looking at the outside, especially if they are base models. There were certain colors and trim packages that were only offered for certain years but that's for the "senior level" class lol.
75-77 have the newer style steering wheel with the rubber horn pad as opposed to the 66-74' chrome horn button.
76' and 77' had the "Y" steering linkage, very distinguishable if you look under the front end.
77 was the only year for the fuel doors and the vertical rear fender lights.
There are so many other year by year quirky little differences but I won't get into those, and remember all of this is contingent on the bronco still being in pretty original condition.
There's all the hints you need to figure out what two years the bronco could be in the pic. Focus on the fenders and the fact that you CAN'T see the front. :) :)
MOD EDIT:
Hope you don't mid but I went ahead and added the original pic I posted to your post. So no need to keep scrolling back and forth for anyone that wants to try to figure it out. .
View attachment 29741