Auction Series 257-264 in Zurich Day 4

Coastal Ships - Helsingfors Area - Skärgårds Trafik Aktiebolag (STAB) 1886: 10 penni yellow & blue, Stone I, perf. 12½, a used example in combination with Finland 20 p. orange on 1886 cover to Ekenäs, the STAB adhesive cancelled by manuscript pen cross and by 'Helsingfors' cds (Aug 25) in black with repeated strike below and faint 'Ank.' arrival at left (26/8). Illustrated and discussed in Dr. Connell 'The Letter and Parcel Stamps of the Finnish Shipping Companies' on page 40. Extremely rare and the proving cover for the issue date of the adhesives.
Catalogue 261: Stamp Issuing Steamship Companies