Rob is right. The manufacturers were all (except Enfield) private contractors who's profits were presumably being nibbled away by the Government - as holds good today! As the labour and parts costs decreased you can bet that the unit price decreased accordingly. So they made hay while the sun shone and used up all the parts they had in stock as once the parts are declared obsolescent, no more are ever ordered * so if you've over produced, you've dipped out unless you use them up!
Same at H&H and Sten gun parts manufacturers. Paid up to a certain date and then STOP. What you have left over is tough luck!
* Only exception to this rule that I know of was the L4A1 mag catch with combined HOD that was re-ordered so as to make use of old, needy but otherwise serviceable L4A1's in Ordnance stores