Official answer from Panasonic DE (just asked the customer service):

With reference to your enquiry, we can inform you that these are identical devices with the same technical features and functions.

The letter "S" is the designation of the colour and in this case stands for stainless steel. The additions "EPG" (e.g. designation for continental Europe) and "WPG" designate the production sites or places of distribution, but this is not relevant for the function of the microwave.

Some dealers include the complete designation in their offers, others leave it out. There are no guidelines for this.

There are two different models which differ only in the materials used and the number of operating levels.