It looks to me like a fixed head body has been fitted to a swivel soleplate machine. The upright locking mechanism differs if that is the case. Have you rebuilt it with a donor machine?
Although, this machine isn't designed for hard floors, and is about twenty years old now. As you mention hard floors, it might be time to upgrade it.
If you can post a photo of the underside, I might be able to offer something more constructive.