One thing to watch out for is the very common ‘rattle’ in SBLOO finders. It’s the glass coming loose within the housing. You might be able to tighten it all up by pushing (carefully!) on the front piece of glass. I did that with mine and it seemed to work. Of course it may happen again, but something to bear in mind.
I use a SBLOO on my 246 and M-D and love it. As an eyeglasses wearer, I find it much easier to use than the 35mm frameline in the camera.