With apologies for the multiple postings, I did want to answer Jerry's query:
Was beach-plum jelly really available in the U.S. in those days?
I've never heard of it for sale. Is its presence a sign that
the couple spent more than they could afford on their present?
Here you go. By coincidence it's from Cornell: http://www.beachplum.cornell.edu/bpguide.pdf
In the 1800's, several attempts were made to produce beach plum cultivars with outstanding fruiting
properties. J. Milton Batchelor, who worked for the Hill Culture Division of the Soil Conservation Service
of the U.S. Department of Agriculture, scouted the east coast for high quality varieties to bring into
production. Further selections were made after World War II by members of the Cape Cod Beach Plum
Growers Association. This group selected and named several cultivars, including 'Eastham', 'Cotuit', and
'Wheeler Sensation #6' (7). As far as we know, these selections have been lost.
In the 1940's, there was a resurgence of interest in the economic development of the beach plum,
according to horticulturist George Graves of Martha's vineyard, Mr. Graves, writing in National
Horticultural Magazine in 1944, said enough was known about the plant to "warrant planting Prunus
maritima on a considerable scale, and for itself alone, since its fruit flavor is unmatched by that of any
other fruit known to the jellymaker or fruit preserver." At this time, the Arnold Arboretum of Harvard
University and the University of Massachusetts began work to develop the beach plum in the hope that
this would lead to the development of a small fruit industry on Cape Cod. Cultivars were selected,
diseases and pests of the plant were documented, and propagation and cultural methods were
developed. On Cape Cod there was a great deal of interest in the beach plum - in growing it, improving
its yield and harvesting its fruit. In 1841, 15,000 bushels of beach plums were harvested in Barnstable,
Dukes and Nantucket counties (2). Bertram Tomlinson, the Barnstable (Cape Cod) County agricultural
agent, reported in 1948 that the making and selling of beach plum jams and jellies was a sizable
commercial activity based mostly on Cape Cod. "Much of the Beach Plum jelly is made by thrifty
housewives, who discovered that a tidy sum could be realized by selling direct to consumers at modest
road stands, " he wrote, "but a few commercial concerns have also found jelly to be profitable, and their
products are distributed throughout the country."
Given this, it seems likely that VN ran into Beach Plum jelly while living in Massachusetts in the 40s. And it also likely means that the gift pack of jelly jars in S&S originated in Cape Cod.