Custom Reply button

I'm attempting to build a custom message reply button and came across some ID's that I'm struggling to identify.

For example:

A post like this: 

... /t5/CRM/Great-test-subject-line-here/m-p/60#M1

Reply button like will look like this: 

... /t5/forums/replypage/board-id/sales_CRM/message-id/1

My question is, how do I access that number "1" or "M1"in this example via the API or a context object? All I seem to find is the ID of this message itself, which in this example is "60".

 Thanks!