If I remember correctly, not all GR1a's had the SLIR fitted.
On the SAOEU at Boscombe in the mid-late 90's we had a mixture of GR1, GR1a and GR1b aircraft, and the GR1a we had at the time ZG706, did not have the SLIR kit fitted, but had no cannons and was to all other intents and purposes a GR1a.
Unfortunately I cannot give you any specific facts fleet wise, but I can tell you for sure that at least one GR1a was not fitted with SLIR!!
Flipflopman