It appears I have opened a can of worms!
My understanding of the valances is as follows:
Part Number 814529
The original front valance, holes for the front under riders and front triumph badge included, no mountings for a spoiler.
Used on very early Dolomites and Toledos, don't know about the 1500FWD.
Part Number XKC0504
Holes for the front under riders and front triumph badge included, mountings for a spoiler.
Used on early 4 Headlamp shells (Dolomites, Sprints. 1500) and Toledos.
Part Number ZKC2657
No holes for the front under riders and front triumph badge included, mountings for a spoiler.
Used on Dolomites and very late Toledos, introduced sometime in 1975 / 76
Most new old stock valances are likely to be these as they continued to be made sometime after production stopped.
Regarding the lip or lack thereof for mounting the spoiler, my Toledo is fitted with part number XKC0504 (November 1974 build), so the middle of the three known valance variations, it has the under rider holes, badge holes and mountings for a spoiler. If you look at it closely you will spot two design elements which I think are pertinent, firstly when looking from the wheel arch down on to the valance wing join the valance protrudes approx 25mm further back horizontally, secondly if you look at the valance from below corresponding to this extra 25mm there is a swage line running the width of the panel. I have included a couple of photos of my Toledo below to illustrate this (both panels are the original items on the car).
Please excuse the dirt, but I believe in using my car all year round and it was cleaned on the weekend.
If you look at period photos of the very early cars (J reg or thereabouts), I have not been able to see the swage line running across the width of the valance, and it appears the valance ends in line with the wheel arch. I could be wrong but I believe the valance was extended back approx 25mm, and the mountings added for the spoiler to change part 814529 into XKC0504. This I believe is why the valance protrudes further back in relation to the wing than you would expect, rather than retool the wings Triumph just added a chamfer to the edge of the valance.
The next problem is when was the change made, and what is appropriate for a K reg Toledo, it looks to me like the valance in this picture below if you extrapolate the edge to take into account the missing section would be in line with the back of the wing.
What do others think? I would probably leave the unit fitted alone though and not try to replicate this feature however.
Edit:
Just found this on line, it looks to me that the valance lip is in line with the wheel arch (click to enlarge):
https://www.flickr.com/photos/31411679@N08/34583171170
Someone should have word with them about their sidelight / indicator units.