What do you mean by "below it"? My catalog 00-5076 (nb., not a typo; one digit lower than in your post) shows 66-1684 for the timing cover spring, as does catalog MC714-3 (Aug. '55). I can't find the second number you listed, i.e. 67-1496, in either catalog. The list of part numbers in numerical order at the back of 'The Gold Star Buyer's Companion' that was created from a scan of the catalog does not show that second part no.
It looks like we have another mystery. With one of my employees looking over my shoulder (making sure I'm not going senile), I'm looking at BSA Spares, Gold Star (500cc.) (Red cover) Catalog No. 00-5077 Printed in England Sept. 1958 Page 17 (engine section) #46 66-1684 -- 67-1496
Coming to terms with your delusions, is the first step to sanity
Now that I know it exists I'll have to start searching for a catalog # 00-5077. Sigh... However, first I'll search what I already have since there's a 50/50 chance I already have one but I never noticed because the covers are identical.
My version of the same parts book above (red cover, # 00-5076, printed Sept 58) has the same #46 - 66-1684 and "-- - 67-1496" detail.
Looking at the 1954 '350 cc and 500cc Single Cylinder;inder O.H.V. Models' book (green cover) the ball doesn't have a part number - it's merely listed at '1/4" diameter...Oil valve spring ball' the 'Oil valve spring' is listed as part # 66-1684.
The big-fin parts lost in the ubiquitous BMS book reflects the red parts books above (with assumed errors). The late ZB/BB lists further into the book reflect the same details as my green B31/B33 book.
No generalisation is wholly true, not even this one. Oliver Wendell Holmes