I have written about the 3M before. In my automotive work, I have used it for years on plastic and rubber parts. It goes in and rejuvenates, brings back flexibility and color. Some will say it does no good on celluloid, but I am a believer. I have many rot-prone scales in my collection. I treat them once a year. I clean the scales with diluted simple green and a toothbrush, after drying well, I slather the scales inside and out. Let it sit on a hard surface, turn over a few times. Repeat next day. Let sit another day.There should be little excess to wipe off after. Makes the colors pop again. Seems to keep the rot at bay, for me, anyway! ;)
Attachment 176182
As to the crack, I would suggest taking the blade back past the crack in original configuration.
A razor like this with a barber's notch would not look right, JMO.