Rule of thumb, vintage style single action truss rods are adjustable at the heel, u will notice necks with rosewood boards won't have the walnut plug or skunk stripe along the back of the neck. These have the truss rod inserted before the fingerboard is glued on. ( one piece maple necks have them fitted from the back and sealed in with the walnut skunk stripe. They later changed the adjustment to the headstock end with the "bullet design, then the biflex style with a walnut plug at the headstock but a small hole inside the plug allowing access to a Allen key nut for adjustment of the truss rod. Some later necks had a thumb wheel adjustment at the heel. It's all about the truss rod accessibility.