Why did Nikon F2 have horizontal shutter?

Brett Rogers said:

Under the circumstances I'd say that it's more like a pre-release than conventional lock up, and hence, when I suggested previously that the Contarex does have a MLU, this isn't really correct, yes? Sorry about that.

A few years ago I was entrusted with the sale of a small collection of Alpa cameras and lenses. Actually I kept some and enjoyed using some of the others. There was a Reflex and also a Prisma present. The Prisma mechanism worked and I enjoyed inspecting and testing it including its mirror lock up. You can see some images at various angles of both, some of which give a good view of the mirror lift lever that sits close to the front of the finder on the wind side of each, in these albums:
https://www.flickr.com/photos/43224475@N08/albums/72157679468395771

https://www.flickr.com/photos/43224475@N08/albums/72157677723061994
Best Regards,
Brett

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I would agree that the Contarex is not a "true" MLU function as we know it today. I suppose in that implementation every other camera would also be considered to have MLU!

Thanks for the Alpa photo collection as they look great!
After doing a little research into these, they are truly elegant which of course is to be expected from a camera that is hand made by Swiss watchmakers!
But yet here again, is it truly MLU? In this YT video of a perfect specimen later model Alpa Reflex Model 6 he demonstrates the mirror locking up part at about 4:24.

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It appears the mirror lockup is again part of the sequence of firing the shutter. That it stays up suggests that it's not an "instant return mirror" either since you have to release the shutter for it to drop down. Since this appears to be a later model then perhaps the prior models are also functionality the same? If so then would you agree that these Alpas would not qualify as a true MLU either?