unit numbers

From 1830 (when I turned on the stream) until posting this, I have heard Berks giving security checks on P-1 to a "Unit 8-7-5-3" or "Unit A-7-5-3". Not Eighty-seven - Fifty-three, but Eight-Seven-Five-Three or A-Seven-Five-Three. Who is this? They were somewhere (not mentioned) awaiting...