I was so blinded by the socks, I missed this other anachronism in Broadway's LOMBARDI:
On the right, the actor playing Jim Taylor is wearing what appears to be an off-the-shelf TK throwback helmet, manufactured by Riddell and widely available from collectibles stores including the NFL's online store and the Packers Pro Shop. If not exactly what Taylor wore in 1965 (right), it's close. Certainly close enough to give a feel for the period.
Robinson, on the other hand, is clearly wearing a modern face mask on his helmet. His elaborate cage looks especially strange next to Taylor's double-bar Riddell. Maybe not the 2010s, but perhaps at least the 1980s.
In 1965, a lineman's facemask incorporated a single vertical bar. Maybe not as intimidating as the bizarre spiderwebs worn today, but I don't think anyone found Robinson or Nitschke any less intimidating for it.
Here's a picture of Robinson at Lambeau Field on January 2 1966, just weeks after the play is set. He's going after the mighty Jim Brown in the 1965 championship game. Although Brown would come down with the pass, the Packers won the game, and the world title, 23-12.
Helmet Hut has a beautiful 1968 Robinson helmet on display.
This facemask is slightly different facemask from his 1965 version, not as curved at the corners but still sporting a single vertical bar. This photo of Robinson comes from about the same period:
We can narrow down the date through the opponent: he's chasing running back Tommy Mason, who was only with the Rams for three seasons, 1968-1970. In 1969 the Packers wore an NFL anniversary patch on their left shoulders, so that would make this photo either 1968 or 1970.
I'll see if I can figure out exactly what facemask the Broadway production is using.