Lot 304 at this week’s Holts is something I have not previously encountered.

[Linked Image from i.imgur.com]

It is a multi function mould and tool to re-load the .300 Rook Cartridge.

[Linked Image from i.imgur.com]

It is clearly set up to de-cap Boxer primed cases, although Kynoch factory loads for the last 100 years or more have used Berdan primers ( which I de-cap hydraulically). Remington UMC did make the cartridge, no doubt with Boxer primers.

Functionally very similar to the Ideal No. 4 mould and tool it also incorporates at stab crimper for the loaded round.