For an RB67 back, the most practical solution is usually to buy another back or a cheap damaged one to use as a parts donor. These backs are old, and specific small replacement parts are generally hard to find. Based on the answers, this particular spool-retaining piece is not likely to be an easy factory-replaceable part.

A DIY repair may be possible if the missing piece only needs to act as a simple round support for the spool to rotate on. Some people would try shaping a small replacement from modeling putty, Sugru, or a cut plastic disc, then attaching it carefully. That could work as a functional fix, but it would be a custom repair rather than a standard replacement.

Also, don’t assume RB and RZ parts interchange; the answers specifically caution that RB and RZ backs are not interchangeable.

So in short: if you want the most reliable fix, look for another RB67 back or a broken donor back. If you are comfortable experimenting, a small DIY repair might be worth trying.