It all started with the introduction of the Dynoshift Series 62 lathe in 1955. By 1960 Monarch wanted to sell the Series 62 with a more conventual headstock, thus the 32 speed Series 612 was born that only differed from the Series 62 by headstock. The Series 610 was the economy version of the Series 612 offered in narrow beds. The Series 613 was a combination of the 612 and 610 moldes in the 1970's, and the Series 614 used just the 2013 and 2818 moldes that trace their way back to the Series 612, starting in the early 1980's. Thee were some improvements made along the way. Motor outside the leg, motor inside the leg, inch/metric gearbox, digital readouts, etc. But dependent on the model they are all basically the same lathes.
Interesting, one of the last major orders that carried Monarch for several years was a large U.S. Navy order for 614's. they had inch/metric gearbox, 2000 rpm spindles, taper attachment and digital readouts. Monarch build close to 100 of these lathes from roughly 1990 through 1997, shipping 1 to 2 a month. Most where models 2013 x 54, but there were smattering of 2018's and 2013 with longer center distance of 78", 102" and 126". I think the longest shipped in this order was a 174".
John L.