The earliest High Pressure Sodium lights in the Twin Cities area were on I-35W between 46th and Lake, I-35E north of downtown, and I-94 between them. These would have been no later than the late 1970s.

These were not any bigger than a normal cobrahead light, but had cone shaped reflectors. You can see some in the backround here. Any ideas what these could have been?

I have seen these, they are some sort of NEMA head but the glass lens is conical, these are very rare now.