Joined: Jan 2010
Posts: 9,835
Likes: 1,816
From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
I didn't catch whether the OP is using a 6-speed Suntour Accu system(?).
The spacing between cog centers is the same for Shimano SIS-6 and Suntour Accu-6. Thus, the two are interchangeable.
Suntour's 7-speed freewheels (and U-6 freewheels) is where the bigger cogs were spaced more closely than the smaller cogs, so there is a visible difference there from Shimano's 7-speed freewheels and cassettes, and interchangeability between brands is what you would call "class B" as far as index compatibility.