Thanks for these Drew.
It looks like there’s a cutoff burr in between the aluminum ring and the washer that didn’t quite get cleaned out, that could be an indicator that they cut this ring themselves from round barstock.
Based on the burnishing marks the steel (definitely rusted) and the end of that aluminum emitter end cap must have been sanded at the same time. Perhaps it was a tapped and press-fit one off? In that case- it is a strong argument for mild steel. If it were stainless I don’t think it would be quite *that* rusted and if it were a zinc plated washer I don’t think that burr would be present even if modified.
(Just positing theories if not already mulled over)
*edit* however the inner and outer rings of the washer are bright still: so perhaps it *was* a coated washer? Just not sure how that burr would be present then…
View attachment 1695956
(This rusted burr)