I'd guesstimate that's fairly early production - 1988-90 or thereabouts. Since Webley junked their records when they went bust in 2006, I don't think accurate dating is possible from any resource I know of.
I think the Pro-System (of which a Chinese copy was still available recently) dates to the early 90s (others may know more), so not a bad choice. Others would be any of the classic 80s suppressors - Galway, Gunsport Hushpower, or contemporary alloy knock-offs of the Venom Slimtech. The actual Venom thing would never have been fitted to a budget gun like an Excel.
It could look and handle quite nice fitted with a Tracker barrel weight, if you can find one.
Always thought Webley missed a trick in not using that item across the springer range to offer a scope-only model - barrel weights and not very effective suppressors were the 80s airgun version of "tacticool".
Also never understood why the Tracker (and Viscount, as the original black-stocked "Stingray") got options like a matte black or camo stock, but the more popular Vulcan-based range didn't. A "tactical" stock barrel-weighted Vulcan/Excel would have sold.