This stained registry cover was probably used to send stamps from the Antigua Post Office in St. Johns.
St. Johns is on the northwest coast of Antigua which is in the
northern Lesser Antilles at the east end of the Caribbean.

The cover has two St. Johns CDS and two oval POST OFFICE ANTIGUA handstamps.
On of the oval handstamps was initialed by a clerk.
There is also a faint ON HIS MAJESTY'S SERVICE handstamp across the top.
The cover was given a preprinted registry label and send on its way.

It is interesting to note that there is was no blue crayon
cross common to Commonwealth post office registry marks.

The letter proceeded to New York where it arrived on
October 3 and was given two oval registry stamps.
Again it is interesting that the USPO didn't add a registry number.

It arrived at its destination of Penns Grove (caution slow loader), New Jersey, on October 4.
Penns Grove is on the east side of the Delaware River across from Wilmington, Delaware.
Before the days of the interstate highway system,
the Penns Grove Ferry (caution, slow loader) was a principal Delaware River crossing
for automobile traffic flowing along the east coast on Route 40 from 1927 to 1929.*

The cover is franked with King George V stamps from the 1921-29 series.
There is a 1d bright violet issued in 1923 and a 1/ black on emerald paper
which is most likely the multiple crown and script CA watermark issued in 1929.
The former is SG 64 and the latter is SG 76.

*Petrelet (Pete Kimball) makes this interesting surmise regarding the addressee: