Ben, my original reply (e-mail) came back to me undelivered because the inbox was too full, so I sent it a second time, and it still came back undelivered. Here's my original reply:
"Are you asking when DeTomaso Modena was founded, or when DeTomaso Automobili became a public company (S.p.A.)? There are several DeTomaso companies, these two being of most interest to DeTomaso automobile owners.
My best guess, DeTomaso Automobili would have become public in 1967. This is the year when it is obvious DeTomaso or Rowan decided to do something "big" with DeTomaso Automobili. This is the year they purchased Ghia & Vignale, and they constructed the new factory for Mangusta production.
DeTomaso Modena was most likely founded in 1973 or 1974. The DeTomasos sold their remaining interest in DeTomaso Automobili to Ford in 1972, and walked away from the Ford deal either that year or in 1973. They had a pocket full of cash, went off to salvage Benelli & Moto Guzzi & later Maserati & Innocenti. Ford wrapped up Pantera production around August of 1974, and sold the unfinished coaches to DeTomaso. DeTomaso built a new factory and resumed Pantera production shortly after that under the name DeTomaso Modena, as Ford still owned the rights to DeTomaso Automobili. DeTomaso later purchased the rights to DeTomaso Automobili back from Ford for pennies on the dollar."