Hi PB. Blondie seems to be the expert in digging up this stuff and I am definitely not but let's see if this is of any help.
Try the July 15, 1976 Watchtower for a possible mention of failed expectations (my understanding is the WT refuses to accept any blame in this misssed date) BUT also compare that with the March 15, 1980 of the Watchtower (I understand from reading Ray Franz's description that the org is more forthcoming about their measure of fault in this article).
I hope this helps. I got these magazine references from Ray Franz's book Crisis of Conscience in Chapter 9 where he candidly describes the 1975 (false) prophesy as a formal inner circle participant at the Watchtower Bible and Tract Society.