On my last trip to the UK I got this strange stamp. I have the feeling I have been flagged but not sure why. Does anybody know if it is something I have to worry about?
I am a US citizen.
Rather than an 'Open' date stamp (an all-in-one) that grants six months, the Officer has done a "coded" landing". This means he/she was not 100% confident you were a genuine visitor, but landed you anyway. The ref number relates to the ref number on the Landing Card that you completed. This means the card will be retained. On future arrivals if there are any concerns, the Officer can retrieve the info given on your Landing Card of 23 March 2019.
On the front of your card are your personal details. On the rear of the card the Officer will make notes as to what was said re: reason for visit etc. The ref number is on the rear of the card also. Those notes will be available to other Officers on future visits.