Tell me about these cowling fasteners…

I’ve got lots of these. I know what they are, e.g. you will find this type used on the Avro Anson, so they are cowling/panel fasteners dating from around the 1930/40s.

I have identical fasteners with different part numbers, e.g.

Example 1 is stamped S.S./2998/1 (that’s S.S. (over) 2998 (over) 1, while example 2 is stamped S.P./33830/3. These look as though they may be (AVRO?) drawing numbers. Both are also stamped with the same makers/inspectors stamp consisting of a small circle containing SET/1.

Beyond that, I’m fairly ignorant about them. Can anyone clue me in?

I’d like to know who made them, and whether they were used by any other airframe manufacturer (or any non-aeronautical use?).

They look similar to the ones used on Typhoons, Spitfires, etc, but they are not the same. Was there a range of these fasteners, available off the shelf, or are they one-off designs to suit a specific job?

I cannot trace an AGS or Air Ministry Stores Reference for them and I’ve never see anything from Dzus that looks like these.

Here’s hoping the forum can fill in a few gaps in my knowledge!