It is old powder from old ammunition that was scrapped because it was 1) unsafe to keep in storage and 2) likely to cause malfunctions resulting in minor malfunctions to blowups.
There is a long thread about old ammunition that I contributed to here:
Since you have it, use it up quickly. Old gunpowder does not get better with time, it was at the end of a reasonable shelf life when it was scrapped, so it is probably anywhere from 20 to 45 years old.
If you get sticky extraction once in a while, with loads that should be safe, that is a warning that the stuff is having burn rate instability.