I think it meant heaver spring rate and bigger sway bar and maybe shocks. It was a $12 option in '69 http://www.hemmings.com/mus/stories/2005/09/01/hmn_feature10.html
In my 1974 Ford Passenger Car Pricing book it states: Suspension, Heavy Duty Includes heavy-duty front and rear springs, heavy-duty shock absorbers, and front stabilizer bar... Wholesale $9.91 Retail $11.66 Maybe Mercury, just used a fancy name?
I ordered my 71 Comet GT new in 71. It had the heavy duty suspension. On the order sheet it was listed as Handling suspension package. I don't remember the cost. too many moons ago.
Wow... couldn't even but the BOLTS or BUSHINGS for ten bucks nowadays... but back THEN ten bucks was probably several tanks of gas so go figure... The reason I love Mercury's is that they always "fancied up" everything and charged more... Cougar (fancy Mustang), Cyclone (fancy Torino), and of course the 71-77 Comets we banter about here incessantly - fancy Mav- er.... fancy Pinto.