I wondered that, too. I've been past the site of the very first Piggly Wiggly in downtown Memphis, but I can't remember the details too well.
I know - the tone of the whole ad is so dignified and classy, and then
Piggly Wiggly was actually the first grocery store to introduce "self-serve" shopping. Before that you would give your list to a clerk to fill your order for you.
Yup. A number of ads from this period and even later instructed the shopper to "ask your grocer" for the specific brand advertised. A Kellogg's ad from a few years before this -- I'll have to scan it and post it -- shows a child coming home from the store with the Wrong Brand:

When Johnny comes
Marching home again
With a common
Imitation -

Send Johnny a
Marching back again
With a note of
Explanation -

Then, underneath that, handwritten:

Never send me anything but Kellogg's - the Original Toasted Corn Flakes