RIC 18 has a mintmark of TESA, with the workshop being a possibility of A, B, Γ, Δ. Your mintmark is TES though, and the workshop should be in the right field, with the same possible ones of A, B, Γ, Δ; it kind of looks like B. In the left field of your coin it looks like ✶ over M. So your coin is RIC 27b xxx.
If you haven't seen Dane's spreadsheets yet, they come in pretty handy
http://www.catbikes.ch/coinstuff/coins-ric.htmbut in this instance she has the information a bit wrong. She has RIC 18/27 as the same, which the reverse is; but the mintmark is different- TESA vs. TES