The leather cases are made from English cow hides and use a process called "splitting". This divides the hide in two which halves the thickness of the leather; the two halves are then coated with a vinyl plastic to make the resultant leather supple, very smooth and weatherproof. They also offer protection from dust, grease and debris.
Just about any liquid as all liquids conduct ultrasound. Gels are generally used because they stay where they are put. There are different types of couplant, water based which can be a problem on steel as unless cleaned off it tends to make the steel go rusty, oil based which is OK on steel but may stain plastics, and silicon based couplant for high temperature use say over 60 degrees C, 140 degrees F.
So in an emergency any liquid will do, a lot of people use spit.
ABS Classification Society Manufacturing Process Approved
ISO 9001:2015 Certification Number 14124458
IIMS Corporate Membership Number C508
NCAGE Number U0B22
NFPA Membership Number 2800960
FPA Membership Number 27135