There is a record of this man being discharged with service no 3305783, but that led nowhere. There are too many Robert Burns born in Lanark in 1887 to be viable to buy them all to get the one with right dob
1887 Sep 13 Born Lanark
1921 Apr 21. Joined with RIC no 80832 & ADRIC no 379 (734). Labourer and ex-Soldier
1921 May 31 Posted to J Coy
1921 Jul 5. Posted to 4 Gds