Thanks everybody so far for their input. I followed some of the leads but couldn't come up with a conclusive answer.
The Helicol color scheme is similar but different. Also the machines have extensive writing on the tail side(s) and Columbian registration (I found some pictures from the 1960s on the company page).
The notion the picture has been taken somewhere in Latin America (rather than Texas) sticks with me. However, I couldn't find the number in any database, except the Ozark-Blackwell field's listing, which unfortunatly does not provide further information.
If this is – as has been suggested – a US Army helicopter, I still have to find prove for that.
And please, if anybody knows something not yet mentioned ... I am eager to hear it.
Again, thanks everybody ...