In the years before WW2 hundreds of thousands of ordinary horizontal milling machines were to be found employed in every kind of machine shop. Whilst, even in their small forms, these were tough and reliable units, they were not the most versatile of machine tools and hence often stood unused whilst small or complex jobs passed through the shop. To make them more useable one solution was to fit them with a vertical or slotting head, and many makers did offer such accessories for their own machines. However, not all did - and many examples were ancient, with their maker long since departed. It was into this potentially lucrative market that several American firms stepped, notably Bridgeport with their original Model C, the "Tree" from Racine, the "Halco" from Detroit and Kearney & Trecker with their Dalrae-manufactured "Midgetmill" and "Speedmill" units and, rather late in the day, the Marvin made by Marvin Machine Products Company of Detroit.
Like most similar types, the Marvin (probably available until at least the late 1950s) was carried on the end of the round overarm with drive by V-belt from a  pulley attached to the end of the horizontal spindle. To change the direction of the belt run - and to tension it - a pair of jockey pulleys were employed.  Well engineered, the Marvin had a spindle that ran in Timken taper roller bearings and was obviously intended for serious professional work rather than occasional use by an amateur.  Marvin slotting heads could be either belt or, for heavier-duty work, chain driven - the head's horizontal shaft being driven directly, pulley-to-pulley, with a single jockey pulley used to adjust the belt tension.
Marvin also made a range of other small, high-quality engineering accessories including milling and surface grinding machine dividing heads; rotary tables; micrometer boring head,; collet attachments and 3 and 4-jaw chucks including a precision 3-jaw "Adjustru" type adjustable to an accuracy of 0.005".
More Marvin here