Go to the Fulton Armory Website and I think there's a writeup on the history of the "Tanker." Also, if it was fabricated by Fed. Ord of CA, stay away. Fed Ord was known as the "sparks & chips boys" for their brutal way they'd churn out guns. Their conversion is strictly a parts gun.
In short, only a few shortened Garands were made. The genesis was in both jungle fighting where a shorter gun was handier or for paratroopers (for obvious reasons). The name, "Tanker" was a marketing title coined after the war to sell guns. It works.
Returning to the shortened Garands, a field expedient conversion was sent to Springfield Armory where John Garand examined it and made a couple. These were tested and while short and handy, had terrific muzzle blasts and the idea was dropped. If you go to Springfield Armory National Historic Site today, they have both a field expedient conversion and a Springfield Armory manufactured one on display.
To make a shortened Garand requires modification of the Op-Rod or fabrication of a new one. You just can't shortened the existing rod as the rod length and barrel length is integral with the timing of the function (unlocking, feeding, locking). Gas port size may have to be changed to accommodate the faster drop in gas pressure (thanks to the shorter barrel). Another modification could be in the cutout of the op-rod where it engages the locking lug of the bolt. Cut it a little deeper or shallower will also affect the time it unlocks.
Problem is with these field expedient models, while they could work, in time they could pound themselves apart if not properly engineered. About the best one made was the Beretta version where an entire new op-rod was fabricated. Check out this month's (June 1999) issue of the American Rifleman for a short blurb on the Beretta version in .308.
Enough blabbing for now. The issue is covered fully in Scott Duff's book, "The M1 Garand: World War II."
If you have any other questions, please post it and we'll try our best to respond.
------------------
Vigilantibus et non dormientibus jura subveniunt