Mode Zero

Can anyone tell me what the theoretical centring for early Futaba Servo should be? I know later J plug ones ones are 1520us but the early ones have a square head yet on a report on the Futaba 6 channel M series, the centring is quoted as 1350us where a later report on the FM M series says 1300. Since they both use the same servos, which one should it be (or in between)?

It's alright. Found it on some old Futaba paperwork. 1310us.