I can't read (or even recognize) Hindustani, but this photo seems to match an item from the March 1945 Organizational History Report of the US 771st Chemical Depot Company (Aviation). They reported having erected a monument with an inscription in Hindustani to “serve as a warning to the civilians for the next 25 years that war gases have been disposed of in this locality.”
The 771st was operating Ondal Advanced Chemical Depot in India, which was the central stockpile for toxic chemical weapons in the China-Burma-India Theater. The tens of thousands of bombs on hand included lewisite, phosgene and cyanogen chloride, but mustard agent was held in the largest quantity and caused the most problems with leakage. The bombs were never used, of course, but one training film from that time contends the US was capable of producing in one day during WWII as much mustard agent as the US produced during all of WWI. Although widely called mustard “gas,” it is actually an oily liquid that spreads as a fine mist when the bomb explodes. Unlike true gases, mustard agent is “persistent” and remains active for long periods of time.
The Organization History Reports for the Chemical Depot Companies were kept secret until 2009, when they were declassified under a broad executive order. My father was a "toxic gas handler," attached to the 771st at the end of WWII when the remaining task was to dispose of the stockpile. Most was dumped into the Bay of Bengal, but in September 1945 15,000 leak-prone M47A2 mustard-filed bombs were judged too dangerous to transport and were buried on site. Japan is still finding and disposing of the WWII chemical weapons it buried in China and the US is still disposing of its WWII stockpiles in the US. It is not clear what has happened with the chemical munitions buried at Ondal.