VERIFIED SOLUTION i

MailStream Plus - How to indicate 'Ready to Pay' status in the Mail.dat files, using sub keyword MDMFDD

To indicate 'Ready to Pay' container status in the Mail.dat field code CSM-1124 use an RP INF
(or EP INF or BATINF) with the sub keyword MDMFDD.
Note: the data area (which starts in position 35) first has a 10 byte field for container ship date, followed by a space, so the 'R' to indicate ready to pay would go in position 46. 

With an RP INF, it would look like this:
RP INF                     MDMFDD            R

....+....1....+....2....+....3....+....4....+.


All MailStream Plus (MSP) parameters are listed and discussed in the MSP Reference Guide.  
UPDATED:  April 13, 2017