Yes, they are (fired) shotgun slugs. More specifically, the "vertical" raised ridges which encircle their lead body mean they are the type made for use in a Rifled Shotgun. "Rifled" means the inside of the gun's barrel (the "bore") has multiple grooves which have a slow spiral, causing the bullet to spin on its axis (like a well-thrown football pass), greatly improving the projectile's accuracy and range. Most shotguns have no rifling-grooves in the bore (thus they are called "smoothbore" guns), because they are intended to fire only buckshot pellets, which cannot be made to spin on their axis.
The fact that the raised vertical ridges on one of your fired slugs are smushed flat means that although it was made for use in a rifled-shotgun, it was fired out of a smoothbore one. The owner wasted the extra cost of that rifled-shotgun slug.
Rifled-shotgun slugs first appear around the turn of the (20th) century, perhaps plus or minus about 10 years. Here's a couple of photos showing modern unfired ones for sale.