The following notice appeared in the NRA Journal, Vol. XLIX, No.1 (Spring 1970)
From: The Director, Inspectorate of Armaments.
Cartridges. S.A. Ball. Revolver .380" Mark 2z
It should be noted that whilst the dimensions of cartridges of the above description are similar to those known commercially either as .38" S&W or as .38" Smith & Wesson Revolver and that they will chamber and fire in commercially made weapons their use in other than Service No.2 Revolvers is not recommended unless the weapons in which they are to be fired have been submitted to Special Definitive Proof in which event they will bear a marking in excess of "3½ tons" customarily found on weapons proved for .38" S.& W. Not only is the pressure generated by the Service .380" Mk 2z cartridge some 50% greater than that of the .38" S & W round but, because the bore size of the Service No.2 Revolver is greater than that found in pistols chambered for .38" S&W a considerable loss of velocity and accuracy may arise from its use in commercially produced weapons.