You're right, "Y" and "Z" meant A-side and B-side for Capitol records, see also my comment on Capitol 1010 (from mid-1947 to Oct 1951), like "X" and "Q" for Aladdin, "AA" and "A" for King, "A+" and "A" for SwingTime, "XSP" and "SP" for Specialty.
I would like to register my opinion that the Y and Z which appear on Capitol recordings from this time frame in a position traditionally reserved for take designations (i.e. appended to the matrix by a hyphen) are actually side designations. In support of this, I offer the following: I have never seen two Y's or two Zs on any one Capitol disc. It's always one Y and one Z. You never see any other letter of the alphabet put to that use by Capitol in the runs where the Ys and Zs are seen. If Capitol had, in fact, instituted a take designation system using the latter part of the alphabet, I would expect them to need and use a larger group of letters than just just Y and Z. I doubt that Capitol was ever so efficient in the studio that it it never needed more than two takes or that it would install a system that could handle only two takes of any given track. At the very least there would be an occasional X, perhaps also S, T, U or V. There being no more than two sides to deal with, the Y/Z system makes sense as side designations, but not so much as take numbers.