One set of negatives from the HPL was date March 8, 1980, and is 24 shots of people willingly diving into a large vat of jello to search for a car key. The winner is pictured above.
I cannot find much information online regarding this event, but it may have been one of the first of its kind. These still take place today. You can search YouTube to find all manner of folks trouncing about in giant pools of gelatin. They don't all involve giveaway cars, however.
Over at the Houston Radio History site, one commenter's memories include the first Jello Jump of its kind as a fundraiser for the Leukemia Society run by KRLY. Since her memory of KC and the Sunshine Band at the radio would have to be 1977, 1979, or 1980, we are at least in the right era for the Almeda jump to have been the first.
The Leukemia Society stopped using the brand "Jello" in its fundraising attempts after a few years, thanks to a cease and desist from the Jello Corporation that was apparently sent to the Houston chapter, which also makes me wonder whether this is a photo from the first jump, prior to that letter.
If any readers out there have any additional memories of this event, please leave us a comment below!
Edited to add, the above was the second annual jump.
The third was advertised in the South Belt Leader on March 25, 1981 as the third annual "Gello" jump: