Spa local in 1919

This cover was sent on 2 September 1919 from Spa to Spa with insufficient franking (2c instead of 10c) and was subsequently taxed with 20c. The postage-due stamp is one with the town name printed on it (more valuable!).
See another cover sent from Spa on the same day and also with insufficient postage.

Characteristics: 2c+2c single and 20c postage-due stamp Scott J14 Franking of 2c instead of 10c No cancel of arrival Remain of sticking label on back
Estimated value: $150 Value attributed chiefly to the postage-due stamp [Auctioned by Olivier Simons in August 2005]