Along with the black plastic locks, the C had small silver 'buttons' around the shutter and aperture dials that had to be depressed to move either one. I can see them in the photo.
To the OP, the 2.8C was the last Rolleiflex model with a 10-bladed aperture. The D and later had five blades.
$75? That's a gift, not a purchase. Enjoy.