Usage Help On A Cover: Prexies + Certified Mail

I like this cover because of the Liberty-era certified mail stamp and the 1¢ prexie coils. I'm stuck on the usage though.

1955 Domestic usage.

The 15¢ certified mail stamp obviously covered the 15¢ certified mail service. That leaves 7¢.

The 1st class rate was 3¢. That leaves 4¢ to explain:

An extra ounce would only cost 3¢ which would leave a 1¢ overpayment - I do not think likely, but I do not have a better explanation.

Return receipt cost 7¢, so that probably isn't it.

Airmail was 6¢ so help there - also no indication that this was sent by airmail.

There are no markings on the back.

What does anyone think?

First impressions ... Are there stamp(s) missing?
Note the way the certified stamp and the leftmost coil are cancelled with a mute device. Then note the similar small black mark on the top of the certified stub in addition to the blank spacing above it.

That "black mark" on the certified mail label was a piece of fuzz that fell off as I took a closer look. I do not see any evidence of stamps that have come off, but that thought did cross my mind. If stamps have come off this cover, they came off very cleanly from a spot where a cancellation did not tie them to the cover while the ones remaining stayed securely affixed.

Interesting stamp uses, possibly starting with someone in a hurry and just missed fact that they left an additional 1c coil attached (if 2 oz, then 6 coils needed). June 1955 was first month of Certified service, using a stamp ordinarily at the time only sitting in post offices, not business offices until personnel were aware of new service and went to trouble to buy them & keep them in the office. Just an odd mix with the 1c Coil format. (Penny postcard had ended several years earlier.) Were the 1c stamps from a vending machine? etc etc. If a collector had a part roll, and noticed the ink "line" in the coil (a "line pair" starts the strip of 5 at time) and didn't want to separate them, but if they had "need 6" on the mind I don't see how keeping the line pair intact would still lead to an extra 1c put on the cover.