I seem to remember a "merger" or one buying the other, just from reading magazines of the time, as far as the rocket end went.
That, I do more-or-less know - Science education supplier Damon bought first Centuri (original model rocket and LMR company in Phoenix, also with an industrial rocketry supplier RDC) in about 1968, then Estes (Penrose), and ran them both as "separate companies" to the consumer.
Centuri, the original serious rocketry company to Estes kids educational company, then got progressively positioned in the "lowball" cheap alternative, until at one point, Centuri catalogs were printed on *newsprint*. This more-or-less ran all the old hands out of Centuri, Enerjet/RDC was put out of business, and some of the people run out of Centuri started other companies, the biggest being Quest.
Others are SSRS (Small Sounding Rocket Systems, which was a Enerjet/RDC redux, maybe even with the original owner Irv Wait) and Composite Dynamics (the forerunner to Aerotech). The former Coaster/Minimax people went on to form FSI, and various other people who were former Enerjet customers started ISP (industrial Solid Propulsion) and eventually Aerotech, who are now allied somehow with Quest and have the old Enerjet IP somehow.
Estes and Centuri ran as separate companies while model rocketry wound down through the 70s, finally Centuri stopped existing as a separate consumer company in the early 80s - a pretty sad end to what was former the Cadillac of rocket companies.

Eventually (and Larry Renger and Mike Pratt worked for the combined company and could clarify), a former toy company executive Barry Tunick bought all of these limping along businesses - Sterling, Cox, Estes, Centuri, and probably others, and at least partly to try to cash in on Star Wars prequel merchandising. This was not well-received by almost anyone on any side and I am sure that it did not give the sort of return on investment he expected.
I am not sure what if anything happened after Tunick, or if Tunick got a clue, but at least now, Estes is running like the baby-boomer hobby company it probably should have been all along, Cox got sold off for scrap, etc. I don't know what happened to Sterling IP.
Brett