Today in Postal History

This registered cover provides a tour de force of the registry system in 1921.

The registry process began with the use of the 2d. prepaid registry envelope.
Note the use of a red cross marking as opposed to the usual blue cross.
The envelope was also preprinted with a bold red oval with R.

A 2½d. deep bright blue King George V* was added to pay the international rate (SG 79).

The sender also requested acknowledgment of receipt service and a bold oval AR handstamp was added.
Although there is no evidence that there was an extra cost for this service,
there was probably a prepaid postal card attached for the AR service.

A preprinted registry label was applied and two oval Gibraltar registry handstamps were applied.

The script notation on the rear appears to be a return address.

There are two octagonal Lisbon transit marks.
One is dated June 19 and the date on the second is unclear.
Is it possible that the second was a receiving datestamp
and the first was the date the cover was sent to the next destination?

Although is unclear, it appears that the upper left CDS
was the initial Montevideo registry receiver on July 5.

A new preprinted registry label mark was applied to track the letter in the Uruguayan postal system.
I'm not sure what the script 1079 indicates - probably some form of additional tracking or indexing.

The cover arrived at its final destination in Montevideo on July 6 and a nice CDS was applied.
Sucursal indicates branch No. 13.

*Thanks to David Benson for catching my misidentified King.