The Welrod was a silenced, magazine fed Lee-Enfield (bolt) action pistol firing a .45 ACP cartridge.
The .45 ACP is subsonic, and the Welrod’s silencer fairly efficient, making it a very quiet weapon for a firearm.
Actually, the Welrod was a single shot pistol that fired the .32acp cartridge. Spare rounds were carried in the handgrip. It had nothing to do with the Lee Enfield.
You are probably thinking of the DeLisle silent carbine.