To get around import duties several different manufacturers (Kirby, President and Regina to name a few off the top of my head) contracted Switson industries of Welland, Ontario to produce their machines in Canada. In the case of Kirby they changed the first digit of the model number from 5 to 6 to denote a Canadian machine. Also they didn't change model numbers as often so, in this case, while the 616 was the same as the 516 the US models may have run up to 518 or even 519 while Switson was still producing the 516 for Canada.( I have the 615 and 662 in my collection)
**Note Switson also made vacuums of their own design under many other brand names including Switson, Valliant, Viking, Eatonia, Waverly, Coronado, Fairbanks Morse, Watermatic, Bradford, Baycrest, Viscount, Healthcare, Master, and Trendvac to name a few.