If the camera you are talking about is one of these:
Paxette electromatic - Camerapedia it has a fixed 1/40th second shutter speed but a variable aperture with f/5.6 as the widest opening. It appears that you have no manual control over the exposure beyond fiddling with the ISO setting. I'm only going from what I can find on the web so may be wrong here. Various things occur to me.
1. The 50 year old selenium meter may no longer be accurate.
2. Slide film generally, and Velvia especially, is very fussy about getting the exposure correct. Even the 1/3rd difference between ISO50 and ISO64 would be noticeable on the slides.
3. As you have no manual control (as far as I can tell from the web), using the sunny 16 rule is not really an option.
Personally, I love shooting old cameras like these but I would use a film with a wide exposure latitude, and probably not one which is known for great colour saturation using a fifty year old lens. Given how expensive slide film like Velvia 50 is, I'd keep that back for a camera with a reliable shutter and manual exposure control. In something like this I'd be tempted to a BW film like Kodak BW400CN but the top ISO of 64 makes using that 400 ISO film a little difficult. Perhaps one of the slower BW films like Rollei Retro would work better.
Not sure this helps very much!
Best wishes, Kris.