Not quite correct chris. I have inspected, repaired & modified many Hundred's of thousands of SMGs during my Time in Service. I recall through the 70,s-80,s. A vast number had the crackle finish & were L2A3 spec. these were the original weapons bought by the MOD direct from sterling. They only lost that finish when going back to field workshops when the finish was very worn. Known as 'Rust Proofing'. Or 'Parkerising'. when all component's were degreased, vapour blasted & then Zinc Chromate (Parkerising) Finished. They were sprayed with Suncorite 259 heat resistant finish paint. & baked in a specialist over for 30 mins. For the finish to harden. then ALL components & Receiver's were dipped in in OX18 Oil & reassembled.
The L2A3's that the MOD manufactured illegally without a Licence from STERLING Armament co in Dagenham. Did not have the crackle finish from the off. There are books on the debacle with Sterling & the MOD. Where Sterling quite rightly sued the MOD & won Compensation. It Nearly Crippled David Holroyd who Owned Sterling at the time!.....
That is why the STERLING magazine is a different profile / shape to the MOD issue versions. The MOD version is different in profile to avoid Patent infringement. & thus the Crafty MOD got away manufacturing many, many, Hundreds of thousands of magazines. Without having to pay STERLING Royalties!.....