I know you were looking earlier (Feb 4) for a brass replacement for these fittings. All available in US as well as from Guy Broad in UK (going by Jaguar part No.) are indeed steel.
I purchased brass hose barbs in BSPP thread (British Standard Pipe Parallel) from the Hosemaster in UK:
Brass Fittings - Male BSP Parallel Thread
MHP06/10 3/8" BSPP x 5/8" ID Brass Hosetail Qnty 7 for 3.77 british pounds each, or about $4.86 each - better than the Jaguar part.
Being hose barbs, they have multiple ridges where the hose goes instead of the single ridge at the end of the Jaguar part. They are also longer. But they are brass, and they are the correct thread. I trimmed the length a bit, used new copper washers and they have been great.
On my '69 2+2 with Strombergs there are 6 of these fittings including one on back of water pump where heater return pipe connects.