Simon, If it makes you feel better we all have some of these lying around we bought at one time or another. I have two rounds that I keep in my collection just to remind myself of this fact.
In the late 1960s I obtained from Europe two dummies headstamped FN 49 and FN 50. The nickel plated version is relatively common, but these were coated with black and red teflon respectively. About a year later I realized that both black and red teflon was available in spray cans and creating these dummies from the nickel plated originals is a work of moments.
The red one is somewhat scratched and it does not appear to be nickel plated, but rather a dull brown-perhaps from some chemical used to remove the plating so that the teflon would stick better.
What are they??? Perhaps a serious attempt by an individual or organization to see if teflon would make the dummies more functional? Equally they could be something put together to tempt the collection, perhaps me or perhaps the person I got them from???
I have never seen, or heard of another pair like these two but the existance of a couple of more would not be a surprise.
I keep them to remind me not to get too surprised or to invest to heavily in an “exotic” item.