A few nice mold products images I found:
Two free splicing blocks with each reel!
Image by Carbon Arc
Kodak 7-inch (17.8cm) tape reel, early 1960s. Along their well-known photographic and motion-picture products, Eastman Kodak also manufactured recording tape and tape reels.
One distinguishing feature of their tape reels was a splicing block molded into each side of the reel. Using special splicing tape and a single-edged razor blade according to instructions printed inside the box, hobbyists could splice their own tape the same way professionals did.