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If you can live with steel instead of brass, they are still commercially available:
http://www.alemite.com/catalog/detai...standardbutton

But if you must have brass they are available too: http://www.saeproducts.com/brass-grease-fittings.html

But what if they are BSF?

For BSF I would suggest buying the brass ones a size larger, running the threaded portion on a lathe to the appropriate size, then running them into a die. You likely won't get the threads to bottom out, but that should give a good sealing interference fit. A jeweller's lathe could be used to cut the threads if need be. Seems like a lot of work for the fittings, but an afternoon on the lathe should make up as many as one would need for a project like this.

Very possibly the brass fittings will be buffed/polished before installation. The artillery likes those kinds of things.

If you need BSF thread ones just go to Ebay.uk and do a search for Grease Nipples BSF and you will come across a few like this one in brass:

http://www.ebay.co.uk/itm/2-Vintage-...item27f29598eb

I have a bunch (like a hundred) of similar grease nipples. They have 3/8 x 28tpi (or maybe 0.9mm) threads and are 0.47" tall above the threads. They look exactly like the carrier ones but for the threads.
Yours for the asking, PM with address.