From the announcement:
Operation Buffalo began on July 2nd and it inflicted the most amount of casualties on a single Marine Rifle Company of any operation carried out in Vietnam. What started out as straight forward turned into an absolute meat grinder for both sides.
We've decided to hold a sale on part of the 'Nam range! We are giving 25% off the entire North Vietnamese Regular Army Infantry range (excludes the Unit Card Pack and Local Forces) for the rest of the month. If you've been eyeing up an NVA unit that you've always wanted or are even looking to start a new army then this a great opportunity to take advantage of.
Source