Definitely unusual. The penalty overprint made the envelope valid for one ounce first class mail. Any other service or additional weight required additional postage to be added. The cover shown appears to be carried in the mail stream from one post office to another and probably carried either cash or some other fiscal items. Additional postage was ignored (obviously). Perhaps Thomas will weigh in on this as he is 1) active on this forum and 2) co-editor of the penalty overprint catalog.
These envelopes were used for money order business and other mail where "negotiable papers" that could be turned into cash were mailed between post offices. As such it was entitled to receive free registration. I have also seen them used for WW I era savings stamps business.
I am somewhat puzzled why they used a two cent envelope when I think they could have used a free penalty envelope without postage. Hopefully, someone else can comment with that answer, too.
This envelope was mailed shortly after the rate increase from 2c to 3c on July 6, 1932.
The post office demonetized many of the old rate envelopes with this penalty clause rather than uprating them to 3 cents for sale to patrons, and then distributed them to PO to be used in lieu of producing more penalty envelopes.
The post office also demonetized many special request envelopes which had their corner card printed incorrectly, and blocked out the bad corner cards and added the penalty statement. Presumably saving the post office the cost of the envelopes.
Neat use and it strikes me as unusual to have indications of registry service without payment of registry postage (there are specific examples where registry was available with penalty imprints, but it's not routine). This was sent within the St. Paul post office (to a branch) so maybe they could bend things a bit. The postal savings bank dealt regularly with cash and checks, and the postal savings certificate documents evidencing funds on deposit in the bank, so the use of registry is appropriate.