I had a rummage around in some cupboards for some old parts relating to the 1 ohm standards that were produced a very long time ago.
There are a few major differencies compared with the Thomas type.
The NML wire was Evanohm, not Manganin and the heat treatment process of the Evanohm wire allowed much lower temperature coefficient. The TC of the Thomas type was quite high even for Manganin, the process was not TC optimised.
The Thomas was tightly wound on a brass cylinder, but the NML had a self supporting structure, secured with silicon rubber strips*. That solved the main problem of the Thomas type, the very high sensitivity to temperature variations. (A thermostat fault in the early 1970's resulted in abandoning five of the ten Thomas resistors in the NBS legal ohm group. The temporary rise in the oil bath tempererature was around 10 degrees, but that was enough to make the resistors unstable.)
The NML was not hermetically sealed, even though a flange type sealed container was designed.
* Silicon oil bath will ruin the resistor!