This legend was found on the inner carton flap of a Hytron HY69 filament-type power pentode circa 1935:
"ALL TUBES HAVING THORIATED-TUNGSTEN FILAMENTS ALWAYS SHOULD BE SHIPPED VIA EXPRESS. AT THE TIME OF THIS WRITING THE FOLLOWING HYTRON TUBES HAVE SUCH FILAMENTS: ... HY69 ... . SHIPPING OF THESE TYPES AND OTHERS WITH THORIATED TUNGSTEN FILAMENTS VIA PARCEL POST VOIDS OUR GUARANTEE.
I guess they didn't trust the Post Office/Postal Service very much even back then!!