Trying to date sails like the Neil Pryde in the picture can be tough...Here is what I have learned
If it is a distinct model like an UltraNova or a first generation RAF you can fairly easily identify the year as '84 and '85
Look to see if the name of the sail matches exactly what is in the appropriate catalog or size charts for the suspected year (IE is it a WARP Speed or a RAF Speed?...is it a RAF Slalom or a RAF Slalom Lite, etc etc?)
Look to see if the size of the sail matches exactly what is in the appropriate catalog of size charts for the suspected year. For example the Neil Pryde Garda was made from 1985 to 1987 in all sizes but the 6.2 was only available in 1987. These size annomolies can help you determine exactly which year your sail was made.
Look for slight differences in boom and mast lengths. For example the Neil Pryde Garda 5.0 only had a boom length of 165cm in 1985. By 1986 the boom changed to 170cm.
Look for things like where the name (Neil Pryde is located)...In 1989 they started writing it on the top leech (back) of the sail. In 1989 some of the sails also have a zipper on the bottom batten pocket.
Look for the shape of the window(s) in 1985 and 1988 some of the Wave sails had 2 windows
Look at the cut of the sail and see if you can determine panel cut differences between years that are similar. In 1988 and 1989 they introduced the double luff pannel and the additional window
Consider things like if your sail is monofilm - which model and which year did it come out
1984 - The only fully battened Neil Pryde Sail was the Ultra Nova. For the record this was an amazing sail
1985 - Introduced the first generation RAF Wave sails. Often in pink these high aspect incredibly short boom sails have a distinctive look and dual windows on top of each other, 1985 also the introduction of the Garda series. These sails were a more traditional, modern wave sail - distinguishable because the are all dacron contruction. (Note the Garda series rand from 1985-1987)
1986 - this is where it gets tough as the 1986,1987,1988 sails look similar. Neil Pryde introduced the RAF Wave, Wavelite, Slalom lite, RAF Slalom and Warp Speed sail lines. To date your sail 1986/1987 you need to look at the name of the sail, the cut of the sail and also the size, as not all sizes were available in all years.
1987 -The sails from this year are easily confused with the 1986 sails, but there was even greater variety and different sizes. For example the sail above is a 4.9 RAF Speed. In 1986 the RAF Speed was not available in 4.9.... In 1987 there was: RAF Speed, WARP Speed, RAF Wave, World Cup Wave, RAF Slalom, RAF Slalom lite, RAF Garda
1988 -Im yet to track down a 1988 catalog but I believe the NP RAF Wave was modified to include an additional front window (as per the NP RAF Wave Mark Angulo signature model above), not previously shown in any prior catalog
1989- One of the ways you can identify NP sails from this year (apart from name and size) is they started writing Neil Pryde on the back top of the sail. The range and names of the sails also increased : COMBAT WAVE, WORLD CUP WAVE, RAF WAVE, RAF DYNAMIC, RAF SLALOM, RAF SLALOM-LITE, WORLD CUP RACING, RAF SPRINT, RAF SPEED. Monofilm is introduced on some sails.
1990 - This year was the year that the versatile RAF CAM was introduced. Sails in this year had names like RAF CAM Wave, RAF CAM SLALOM, RAF CAM DYNAMIC. We also start to see the introduction of more clear / monofilm sails and the end of dacron / mylar composites
Neil Pryde 1983 Sail Catalog drive.google.com/file/d/1ZCP3lxfZxTrgnZVgH_dP_y1Ucen21r1x/view?usp=share_link
Neil Pryde 1985 (Partial) Sail Catalog drive.google.com/file/d/1PITslj2iqsZecvdv3GmzqIEkshqGM623/view?usp=sharing
Neil Pryde 1986 Sail Catalog drive.google.com/file/d/1U0IIb0uO-J3xx6BmllWYvQe1S55c2dLI/view?usp=sharing
Neil Pryde 1987 Sail Catalog drive.google.com/file/d/1ahVpJYO2QBFfN3YKFadoVuTJu9Qlwq5X/view?usp=share_link
Neil Pryde 1989 Sail Catalog drive.google.com/file/d/1c8tQSCC1hv1FvjZZSxWpOcvrYjgrtTuN/view?usp=share_link
Neil Pryde 1990 Sail Catalog drive.google.com/file/d/1nxwkxvnLXi7-GCFmt_0LkiKabCcRiIrc/view?usp=sharing
Neil Pryde 2016 Sail Catalog drive.google.com/file/d/1ggokyTsfXzYqCHZ8iuBruv4jtND7DyLv/view?usp=share_link