I have a Longines PAF with 12.68ZS movement and am trying to determine the correct balance staff reference. Ranfft comments only minor differences between the 12L, 12.68Z and 12.68ZS. Cousins movement document lists the 12.68Z as the base calibre so I was thinking a Ronda 1636. If any one can confirm or can help with a parts list it would be greatly appreciated