+1 on soaking/re-lubing. No matter how clean on the outside, the original grease often thickens/hardens and should be flushed and replaced.
+1 on Ultra 6, but there's no need to measure the cog spacing to check. If it has 6 cogs and the small cog is flush with the outside of the freewheel body, it has to be U-6. Std-6/Ultra-7 New Winners used a top-cog-threaded-on-2nd-cog arrangement, so the small cog always sits above the outside of the freewheel body if Std-6/U-7.
+1 on definitely checking closed ebay auctions for as near-identical freewheels as possible to check value. My gut says clean New Winners should bring decent money, better than the $17 you'll pay for a new SunRace. But I think wider-range fws bring more $$ than corn-cobby spreads. Closed final auction prices will tell you what you need to know.
The NWN bearing adjustment was a mixed blessing. A good idea in theory, but the retaining/adjusting ring notches were so small and shallow, and removal/tightening torque so high, that even using the correct tool was no guarantee you wouldn't strip the rings and fail.
I'm not so sure about Suntour dropping the NWN adjustability to fit smaller cogs. The NWN could already be built U7 w/12t small cog, Winner/Winner Pro spec was the same. W/WPro didn't get 11t, did they?
FWIW, every change to the Winner/Winner Pro I saw was done to cut production costs, and the Winner/Winner Pro rollout was disastrous in the USA:
1. Weaker single-engagement pawls snapped on ATB-ratio freewheels: beware single-pawl-engagement wide-range fws, the drive pawl can shear under load
2. Plastic stepped spacers led to the 2nd splined cog slipping off the splines; dealers were sent replacement steel spacers
3. Plastic spacers made consistent cog spacing, needed for indexing, hit-or-miss; dealers had to be sent go/no-go feeler gauges to check fw cog spacing on new bikes, and micro-shims to respace if necessary
4. The "improved" 4-prong removal system failed more often than the NWN 2-prong design, until redesigned tools were produced/shipped
5. I'm probably forgetting something else, these fws were a cluster-f**k from Day 1
Ah my favorite C&V freewheel. I ran Suntour Winners (all three versions) throughout the 70-90s. Most of your inquiries have been answered, but here's my $0.02..
1. Even though it looks basically new, it will still benefit from a flush and lube with new oil. That was my routine too, dunk in mineral spirits, scrub, water flush, WD-40 then run oil through it.
2. If you check e-bay, you'll find Suntour FWs have a high resale value. About $50 average! I'd keep it and replace it with a cheap new FW ~$10 if you search...
I think the New Winner with their adjustable cup/cone method instead of shims, is a plus. The Winner Pro reverted back to shims in order to fit a smaller outboard cog for Ultra-7s.
To see if you have a standard 6 or Ultra-6, take a ruler to the spacing...