Thanks for replying. The wine was made by George Heyser of Hagerstown, MD. The Heyser family is still around so I emailed them earlier this week but have not heard back. I have found no further references to this wine, though they may be out there somewhere, so we do not know what was meant by "slight frost" nor when the grapes were harvested.
In researching further, there was clearly a range of meanings for "frost" with relationship to grapevines. Towards the end of the 19th c a frosted grape vine was one that was frozen. There are many references throughout the century to varieties of grapes which improved their flavor "after frost". This was not just the winter or frost grape. Some of these were clearly meant for the table so they probably were not frozen hard. Some people tied bags or netting over the grape clusters so they could continue past the first frost.
The weather in Maryland for October, 1865 was based on temperature readings taken at 7am, 7pm, and 9pm. On October 10, 1865 the high temperature of the month was reported between 80-83 F for Cecil, Anne Arundel, St. Mary's and Carroll counties. The low was recorded on October 30, 1865, as 31-40 F, with only Carroll county below freezing on the 26th and 30th. The weather appears to get cold into November with Cecil County reaching 23 F on November 11, 1865, and Anne Arundel reaching 28 F on November 12, 1865. It then warmed up with both Cecil, Anne Arundel, and Washington, DC reporting highs of 65-78 F on November 17, 1865.
For October 1865, Sykesville, MD reported frost in the Valley on October 3, 1865. October 6, 1865, saw more frost with tender vines killed in the valley. On October 30, 1865 the ice was 0.25 inches thick. Woodlawn, MD reported partial hoar frost on October 3, 1865, hoar frost and a little ice on October 7, 1865, frost and ice on October 22, 1865, and on October 25th and 30th the ground was frozen and puddles frozen over. They saw a little snow on November 5, 1865.
So it looks like frost and ice the first week, perhaps getting colder through the week, of October then it warmed up by the 10th. Frost and ice returned during the third week of October and temperatures continued to get colder until the end of the month.
Sykesville is further west than Woodlawn but still quite a ways to Hagerstown.