IDW Star Trek Comics for January

[Stevil2001]

Evidently it was counted as one episode in the original production numbers. The production numbers for "The Menagerie" are 6149-16A and 6149-16B, so officially both parts were counted as episode 16. I guess the numbering conventions in Hollywood were different at the time, or maybe it was a Desilu convention and Paramount used a different numbering system.

Well, despite being a two-parter, there was only one episode's worth of material to produce, so a single production number makes sense.