In the years before WW2, hundreds of thousands of ordinary horizontal milling machines were to be found employed in every kind of machine shop. While, even in their smaller forms, these were tough and reliable units, they were not terribly versatile and hence often stood unused while 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.. 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, Kearney & Trecker with their Dalrae-manufactured "Midgetmill" and "Speedmill" units and, rather late in the day, the one shown here made by Marvin Machine Products of Detroit 26 Michigan and Canton, Ohio
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 spindle  ran in Timken taper-roller bearings and the unit was obviously intended for heavy-duty work rather than occasional use by an amateur.  The slotting head (bored to accept round tools up to 0.5" in diameter) could be driven either by V-belt or, for heavier work 9rather dangerously) by chain. By removing the front cover the stroke length could be adjusted.
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 that could be set to an accuracy of 0.005". Additional pictures of the chain-drive slotting attachment here and a copy of the catalog and a sales flyer can be seen here.