Anyone know this orchard sprayer?

Hello, I’m new here so apologies if i mess this up.
It’s an odd inquiry, I know, but I’m wondering if any experts can help me discover the model and date of manufacture of an old sprayer.
The only clue I have is a small flat bag, made of a rough fabric resembling hessian, perhaps 170 mm by 70 mm or thereabouts, which appears to have been buried for some time in the soil of what is today a rural Scottish garden. There is some printing on it. The characters “OMA”, “orchard sprayer”, the digit “4”, and what looks like “25 kg” are readable.
If by some miracle you are able to identify the machine and how it worked, and in particular what this bag might have contained, I will be very grateful to you. Again, I appreciate that this must seem a strange request, but the answer could be of importance to me personally.
Thank you so much!

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