These are some tack style heads to stick on the end of some 6D finishing nails. The ones I used were 2 inch nails that were 2.3 mm in diameter. The heads of the nails varied from 2.7 mm to 3.1 mm and this head is an oval within these ranges to accomodate. I slide the nails (sometimes force the nails), into the head and give them a slight twist (if necessary) to make sure they lodge.

The entire tack head is 20 mm wide and 14 mm tall.