Answered by u/awildwoodsmanappears:
Japanese gas grenade.
BE VERY CAREFUL!
Answered by u/canarchist:
OP, you need to contact the local police and have them send the HAZMAT team and/or bomb squad. Tell them exactly what you know about it and what it could be.
More on it here:
Model 1 Frangible Toxic Gas Hand Grenade (SEISAN SHURUDAN) Glass gas grenades were captured on Guadalcanal and in Burma early in the war. Its designation is unconfirmed and is believed to have actually been developed in the 1930s. They were also identified as "T.B. grenades" by Allied intelligence, but the meaning is unknown. These are the gas grenades once employed against British tanks in Burma near Imphal in 1942. They were filled with liquid hydrocyanic acid (AC), a blood gas derived from hydrogen cyanide. These grenades were initially reported as filled with 80 percent hydrogen cyanide (AKA prussic acid).
Update from original poster u/why_da_herrrooo:
Hi everyone, sorry I was unable to respond... Was a longer than expected trip back home. Thank you all for the responses, and the consensus seems to be that it should be nowhere near any individuals. I let [my] uncle know what everyone said it probably is, and he said he's surprised he's not dead.
[That] being [said], he said that the rubber seal had been disintegrating over the years, [and] that's why you can see flakes floating around in the picture... The rubber seal [had] been disintegrating into the bottle, and he had just been re-sealing the sides with glue.
He said he moved it into another location not near his house and will call some officials tomorrow on what to do with it. He's not too concerned in the meantime, as he has had it on his shelf in his bedroom for the last 50-some years and nothing's happened yet.
Thank you all for the responses... He was actually planning on opening it and switching the contents to a more secure container, that's why he actually showed it to me to see what his next "project" was. You all probably saved a life, so I really appreciate it!