In this image the top and bottom entry are obviously not illegitimate, but the middle one could be.
(Or the product of a Smith/Smith marriage 
)
If you take a look at the above image posted by mckha (Reply #13), the middle entry has number 2555/S
This is a good example as I believe an entry with the added 'S' indicates an adoption.
Annie
The addition of a /s after the entry number does not signify an adoption. It means it is a supplementary entry.
See 13m here
https://www.freebmd.org.uk/vol_faq.html#13mThere is no way you can link entries in the GRO birth index with entries in the GRO adopted childrens index unless you know the names before and after adoption.
