Postby silvermakersmarks » Sun Mar 19, 2017 10:24 am
Published Birmingham records for this period are very sketchy, but an identical mark was registered at the London Assay Office in 1880 by Edward Shrosbree & Thomas Penn trading as Shrosbree & Penn. In an 1884 London trades directory they are listed as stick mounters at 5 Featherstone Street, City Road, London EC.
If the date letter is "t" then you are correct with the assay date.
Phil