For all MGA
1G3668 (metal)
1H3364 (new part number, plastic)
      (Com. Engine GD101, Jan.1959)

21H6060 (third in picture, much later issue)

On Jul 31, 2015, Kelvin Dodd from Moss Motors wrote:   "The MGA steel breather 1G3668 was superseded by the later plastic 1H3364 one [during MGA production, and carried over to MGB]. The TR7/8 Salisbury axles were fitted with part number DAM1695 and it appears when Rover took over as Rover/MG the 1H3364 and DAM1695 were both superseded to the new part number 21H6060, which is what is being currently supplied by BMH.
      The images show the MGA, MGB and current 21H6060 versions of the breather. Physically, the current 22H6060 is very similar to the DAM1695 with an increased reservoir area and larger vent cap, but smaller orifice. This design may have been introduced to reduce leakage, which the earlier versions were prone to".

Note that the taller plastic part is not suitable for the MGA gearbox, because the tunnel is a tight fit, and the plastic part will be broken when lifting the engine for removal and installation.

On 23 Mar 2020, Christopher Cox wrote:
"The original axle breather on top of the MGA axle was made of plated mild steel. The position of the breather tucked away under the car is such that easily deteriorates and rusts. In recent times I have only ever seen a plastic/nylon replacement available from traders. HOWEVER ... there is a nice brass breather widely available, as fitted to Land Rovers".
      "The Land Rover Series axle breather is part number 515845. It is generally available, including eBay, for around £2.87 (about $3.50-USD).
      Mick was querying whether this has an internal valve. Having checked by waving one around in my hand and hearing a rattle, yes there is an unsprung ball bearing within the valve, which acts by gravity on the ball bearing. The ball bearing can be viewed from/through the threaded end.

Thought it might be useful to reference the dimensions of the brass Land Rover breather. Diameter of cylindrical part 17-mm Distance across flats of hexagonal part 16-mm Total height excluding threaded part 18.34-mm (this is the height above the axle casing when the breather is fully screwed in) Diameter of threaded part 9.44-mm (do I recall this is an AF screw size?). That being the case it would be of use for a gearbox as well as the axle"?

Yes, same breather is used in gearbox and rear axle.

Addendum, June 7, 2023:
Thanks to Del Rawlins there is now a 3D printable version available, if anyone wants to try printing their own part.
https://www.printables.com/model/455992-mgamgb-gearbox-and-rear-axle-breather-vent