Nice stamp, and thanks for starting this thread.
I was hoping somebody would answer your question by sharing a similar stamp on a document or cover showing how your stamp was used, because I have had the exact same question for years now.
I am not 100% sure what the numbers mean.
I have three examples in my Hong Kong collection, but lacking the full documents, the context is lost by just looking at the stamps. Most likely, as Peter said, it is some sort of revenue usage, but I would like to see for sure how it was used and what the numbers mean.
Here are my three examples, plus another example snipped from a large online collection, all similar to your stamp. I have seen other examples in collections for sale on eBay
Sorry I cannot help you more than this.