The 9X21 was developped by both IMI and Tanfoglio.
To know more on 9mm and the 9X21mm, refer to this link :
http://p201.ezboard.com/fczechpistols82792frm25.showMessage?topicID=119.topic
The version we are talking about here, is the North American introduction wich is almost exclusively used for IPSC. There is no SAMI specs for it and was not offered as loaded ammunition (N.A.) Tanfoglio is the company that made enough brass and guns to make it popular.
If loaded like a 9x19, it will be useless for IPSC purpose. Remember that was before the reduction in power factor.
The original brass imported on those shore were Tanfoglio, imported by EAA.
Never heard or seen any IMI 9x21 brass on this continent.
9x19 major is looking for trouble, will be safer to go strait to 9x21. Like the 9X23, 38 Supercomp and new 38 Super brass, the 9X21 brass is stonger in the head area. Better choice for high pressure load. On top of this, the 9X21 was and is still used in race gun and is a very effective round for providing enough gazes to make comp work.
The best brass to build a gun on is the 9X23 Winchester, the only 9mm brass rated at 55 000 PSI. Commercial ammo is loaded to equal .357 magnum level with a 125 grain bullet.