Vintage Blackburn Medium Semi Hard Under Seat WEDGE Bag. 

HEAVY DUTY PLASTIC LINER KEEPS ITS FORM! VERY STURDY! SLIGHT BLEMISH IN CLOSEUP ON BLUE! HARD TO SEE!

EXCELLENT USED CONDITION! SEE PICS OF ACTUAL ITEM!

It uses zipper for closer. CLIPS ON SEAT RAILS. VELCROS TO SEAT POST!

VERY LIGHTWEIGHT!

BLACKBURN HAS A LIFETIME GUARANTEE ON ITS ITEMS IF IT IS STILL IN BUSINESS!

It is 7 1/2" by 3" by 5".  

It has a second pouch underneath for like your phone or a credit card.