1952 60¢ Registered Airmail Cover To Germany

I'm having trouble determining the usage on this cover. Referencing the Wawrukiewicz/Beecher book on U.S. International Postal rates, it looks like the airmail rate was 15¢ and the minimum registry fee was 40¢ to Germany on Dec 5, 1952. That totals 55¢ leaving 5¢ unexplained.

I don't see a registry service (or any other service) that could account for 5¢. I do not think the domestic airmail rate would need to be applied in addition to the international one, but that rate was 6¢ at the time, so it doesn't quite fit.

Is this just a 5¢ overpayment?

Thanks for any help!

Return receipt ("A.R."). Cost 5 cents on international registered mail 1879-1953.

@paperhistory - thank you! This is the kind of thing that is easy with experience - thank you for helping me increase mine!

Postage and fees are correct.

The foreign return receipt service was free of charge until 15 April 1925. In the errata published in Stampsmarter Tony W has corrected the 1879 year that was published in the book. There was however for some years before 1925 a fee for return receipts on unregistered parcels to some states in Central America and the Caribbeans but no such parcel item is known.