It's a transitional body: a 2000 FC that has a first version of the curtain retraction feature.
I don't know how many were made, but they pop up once in a (long) while.
Don't know either whether they were all made like that to begin with, or were converted when a camera returned to Hasselblad for repairs.
I do know that there are 2000 FC cameras from the same year that do not have that feature.
But it's known that Hasselblad 'mixed' production, i.e. made older models while already also making the successor model.
UR, by the way, is not 1977, but one year later: 1978.