NavList:

I have an astra IIIB I had bought years ago.  I had asked for a traditonal split mirror, but ended up with a whole horizon shipped by mistake on the order.  Taking it as an opportunity to own both, and with a pleasant offered discount for my politely described troubles, I bought the split mirror seperately and now have both. It is not hard at all to change.  Simply unscrew one (two screws) and screw on the other, then go through and do the standard checks on the mirrors to make sure they are properly adjusted, paralell, side error etc. THese checks are totally routine anyway, and no special burden. 

Frankly I never use the whole horizon anymore (it sits in its box as an emergency spare).  Not worth the bother.  It is easier on some sights but I just adapted to the split happily enough for everything.   

I have to say I prefer the split for sure for mixed sights in low light and Lunars. Easier to rock it around and search for things. I learned first on the old Davis Plastic jobs, with both split on the simple mark 3 and the whole on my I think Mark 25..so was always exposed to both.