b. A small cask or keg holding half a firkin (4{half} gallons: approx. 20.5 litres).
1570 in J. Raine Wills & Inventories N. Counties Eng. (1835) I. 341, iij pynnes for caryage of drenk a feld. 1610 Brechin Test. II. in Dict. Older Sc. Tongue f. 90, Thair is sum laikage with tua laiche rynning penis of Frenche wyne. 1699 B. E. New Dict. Canting Crew, Pin, a small Vessel containing Four Gallons and a half, or the Eighth part of a Barrel. 1743 W. ELLIS London & Country Brewer (ed. 2) IV. 293 Powder one of the Balls and put it into a Pin or Half a Firkin. 1814 Sporting Mag. 43 112 He used to have a pin of beer. c1900 Advt. in N.E.D. (1906), Beer in Cask. Discount for Cash on or before Delivery; 3d. Pin; 6d. Firkin; 1s. Kilderkin. 1968 J. ARNOLD Shell Bk. Country Crafts 293 Each size of cask must have its respective size of chive and croze, i.e. pin, firkin [etc.]. 2003 Times (Nexis) 22 Feb. (Weekend section) 15, I make two casks a day, usually firkins, which take nine gallons, and sometimes pins (4.5 gallons) and kilderkins (18 gallons).

OED