How to set up a Postcard job to have valid piece attributes for MailStream Plus

Products affected:  MailStream Plus™
Operating System: All Platforms, all versions
The P ATTR parameter is used to tell MailStream Plus™ (MSP) the weight, thickness, length, height (and/or the volume of mail pieces). 
And P ATTR position 60 is where you tell it special Rate Type information about the mail pieces, such as being a Postcard or Double Postcard.

MSP validates the P ATTR values for postcards, and a mailer asked where can they find the USPS requirements for a postcard size? When the job does not have accurate P ATTR size information, they are running into problems with their Postcard jobs.

To verify Postcard size dimensions you can reference the USPS Domestic Mail Manual (DMM).  It is located here:
Note the DMM references and information below was current as of May 2019

DMM 201.1.2 
200 Commercial Letters, Flats, and Parcels Design Standards
201 Physical Standards
1.0 Physical Standards for Machinable Letters and Cards
1.2 Physical Standards for Cards Claimed at Card Prices
1.2.1 Dimensional Standards for Cards
Each card (each stamped card or postcard or each half of a double stamped card or postcard) claimed at a card price must be:
  a. Not less than 3-1/2 inches high, 5 inches long, and 0.007 inch thick.
  b. Not more than 4-1/4 inches high, or more than 6 inches long, or greater than 0.016 inch thick.
  c. Rectangular, with four square corners and parallel opposite sides. Card-size pieces meeting the standards in 1.2.1 may have finished corners that do not exceed a radius of 0.125 inch (1/8 inch). See Exhibit 1.1.1.
1.2.2 Weight Standard for Machinable Postcard
The maximum weight for Presorted First-Class Mail machinable postcards is 3.5 ounces (0.2188 pound).

Note: there are other specific requirements to qualify a a postcard.  Please refer to the DMM. 
UPDATED:  May 14, 2019