And did it replace the OVS?

The oldest I know of is from the mid-1990s.

And did it replace the OVS?

1993..  OVZ replaced the OVS.  The OVX was not a direct replacement for anything.  It's really in a niche all its own.

How do you tell an OVS from an OVZ? Or are they essentially the same?
Does it make sense that a utility would switch from an OVS to an OVX when the OVX was introduced? Wondering if there's any OVZs in my area I haven't found.
When did OVZ replace OVS?

How do you tell an OVS from an OVZ? Or are they essentially the same?
Does it make sense that a utility would switch from an OVS to an OVX when the OVX was introduced? Wondering if there's any OVZs in my area I haven't found.
When did OVZ replace OVS?

It's not too difficult, if you google OVS and look at the images, then google OVZ.  It's hard to describe, but easy to spot the minor differences.

The first pic is an OVZ,  The second pic is an OVS.  You can spot the minor differences.