Confirm Postage Amount On Washington Bicentennial FDC

I acquired the pictured FDC with a batch of other covers. It's from the Washington Bicentennial issue of 1932 with a Moore cachet (Planty 704-78, I believe).

The cover has a total of 37-cents of postage affixed, which I think is 7 cents more than was required. Using my copy of Beecher and Wawrukiewicz, I think this cover required 30-cents of postage:

5-cents for airmail;
10-cents for special delivery; and
15-cents for registry service.

Is that correct?

Your analysis is fine. Overpayment on philatelic mail is common. I would not get lost in the weeds to over-analyzing it other than calling it a "philatelic overpayment".

Here's 59 cents on a similar airmail/registered/SD cover sent on his birthday.

Thanks for confirming!

Three cents is the Return Receipt Fee for Registered and Insured show to whom and where delivered. While intended by the sender, intended, there is no indication on the face a RR was wanted.

As an aside, the item was sent when two levels of domestic Registration were available, 15 cents and 20 cents (>$50, <$100).

Although the return receipt or various registration levels still don't come out to an exact payment.

Oops, seven cents over paid, not three...so much for the three cent RR. As to the registration, just mentioned the 15/20 as that was the beginning of multi-step registration levels.