I posted a copy of the cover's reverse on the Frajola PhilMercury message board and asked if anyone could provide information on the marking. I was informed that it wasn't a Scranton transit marking as I had suspected but rather was a NYC examiner's marking. The following background was provided by Stephen T. Taylor (UK):
Quote:
Steve, Jeffery Wallace of Canada has an interesting exhibit of these - part of this is copied below. Believe that Nick Lombadi has written an article about these but don't have that reference handy. Steve
This display shows the small lettered postmarks used by clerks known as Examiners of the Registry Section at the General Post Office on Park Row in New York City between 1884 and 1902. The letter code identified the examiner who "was held responsible for the condition of all articles bearing the stamp which he had signed for".
In 1935, Dr. William Evans found no evidence linking the code to the actual name of the clerk, because he reasoned it was unlikely that the same clerk would be employed in the same role over a 16 year period. Therefore, he suggested it was allocated to a particular desk or section. Based on the 500 plus covers seen by Mr. Barr in 1950s, he drew the opposite conclusion, each mark was assigned to one and only one examiner. The rationale being the mark indicated personal responsibility (thus traceability), and not to a position or desk. Additionally, the marks were usually clean and not smudged suggesting personal care not normally found with shared devices which were more prone to damage.
Curiously, these cryptic registry marks do not indicate a place or even a purpose which is common to most registration marks. The mark is a circled letter with a vertically embedded date in the American M D YY date style. Very little has been written about these marks. The exhibitor is only aware of 4 articles published since the 1930s, with only one being the definitive published by Jeremiah Barr in 1952. With such little coverage, these registry letter codes are often mistaken for NYC station letter codes which were operational at the same period. Thus, auctioneers inadvertently attribute these codes to the stations, or omit them altogether.
Another board member, Ginny Nightingale, provided the following link to the Wallace website:
Quote:Jeffrey Wallace has a website about examiner's marks along with examples.
http://www.canadian-expat.com/ 
Posted 06/15/2011 7:07 pm
