JUNE 18
PAQUEBOT
POSTED AT SEA
RECEIVED
18 06 29
SAINT JOHN N.B..

The Image Sleuth reveal:

The Leeward Island 1 ½ d. key plate stamps was issued between 1921 and 1926. The online catalogue (SW.C) lists it as number 62 (Leeward Is.)
I located this postmark on the cover shown below:

So, to understand this cover, it was posted on a vessel that frequents the Islands somewhere on the high seas between any one of the Leeward Islands and Saint John, N.B. However, the person mailing it did not receive a receipt for registration when it was placed in the Canadian mails to be forwarded to Scotland.
But that was a different time, and possibly the traveller was a regular on this route doing business and entrusted the letter to the purser on board, to receive it on his next return visit.
Alternately, maybe a collector took a chance by dropping it with the blue pencil cross on it in the hopes the postal staff would do the logical thing wherever the mail was off loaded.