nice cab there chap, anyway, yep there has been more post's on here about front grille compatability than you can shake a stick at, but to save you wading throught it all here is a short a answer.
(1) Your Cab has a non s-line front bumper fitted even though the cab may be an s-line version.
(2) Non s-line A4 bumpers have a better array of front grilles to choose from, not OEM ones , but third party grilles.
(3) best plan is to look at fleabay as there are a lot on there, also, just do a search on the net for A4 Front grilles ( non s-line fittment) or if it says wont fit A4 S-line, S4 or RS4 and you will be good to go.
and lastly , the reason the A4 S-line, S4 and RS4 grilles wont fit is becuase they are a deeper grille as the grille apperture on non s-line bumpers is a different depth to the s-line.
if you changed to a s-line bumper then you would have to use s-line grilles or S4, Rs4 , which, as most owners will confirm is about the only straight forward option available.
there you go, all you need to know , you can now go forth and purchase the grille you prefer.
hope that helped a bit.
rob