AO-1908, 1981, Fire Pumper 1860’s, Add-on Cachet, First Day Cover, Standard Postmark, SC 1908, 

Add-on cachet is:

A cachet design added to a cover which was originally uncacheted. An add-on cachet should be identified

by maker and date (mine are stamped on the back with the date) so that it is clear that it is not contemporary with the cover.