TubaForum.net

not Miraphone/not B&S/not nickel...

...but a buck apiece...

I'd wager the ten bucks that the thread is M3X.5

' several of these auctions...

https://www.ebay.com/itm/404536252819

(Some older tubas/ carriage rods are smaller than 3mm threads, and some aren't even metric.)

Thank you for the link. Not sure if this would be the same ones but I have purchased the following set and it fits just fine on JP379B.
https://www.ebay.com/itm/225825963226

I've got a junker Miraphone 186 that's missing the nut (like you've got linked there). Any idea where I could get one in that size? Other than Miraphone?