GTC58,
When people say that 13th Month is incorporated into the based salaries, they do so because as part of the settlement in 1999 where A Scalers took paycuts, B Scale Based salaries were grossed up to incorporate 13th month. I cannot recall if all Bases received the increase, perhaps someone else might recall the details.
I don't say this to get into a p!ssing competition, just explaining the jargon.