small wiring mystery

1970 was a bit of first year run for some of the electrical, but not all of it. The larger fuse box with more fuses didn't appear before when, 1973 maybe?
I did a little research. From what I gather, that inline fuse is added circuit protection. 1970 only, Charger, Roadrunner, Dart too apparently. They may have simply enlarged a wire size in the next year model, or they might have even done that during the 1970 model run. So look at your fuse box. Is there a turn signal fuse? I don't think so. Look wiring diagram. Where did signal power come from? In some cities with heavy traffic a driver could sit with left turn signal on for quite a while. Who knows what number of instances appeared. Not enough for a recall.
Back when I worked for Ford, the Mustang, Thunderbird, Escort, could have fog lamps. Those owners that used them only in fog didn't have a problem. The few that wanted to run the fog lamps all the dang time cooked the wiring and the multi function switch in the steering column. Never was a recall. So your fuse holder might have been dealer installed after a fault occurred.