It's not a MK1, it's actually a MKIII series 1 RS Turbo. The grill isnt the only difference, the RST came with a RS body kit, which is a lower front spoiler, wheel arches, side skirts all of which are colour coded along with the grill and rear spoiler and bumpers. The RST also came with a set of RS alloy wheels, RS decal on the side and front Recaro seats.
Pretty much all of the RST's where produced in white, with the exception of a small number in red and an even smaller number In black.
I'm not 100% but I think only the RST and the RS1600i came with rear opening windows, were as the XR3/3i's where not openable.
Both the XR3/3i and the RST shared the same CVH engine, albeit with different internals