Okay so I was playing around with modding a regular 1/4 drive bit holder to use as a center drive attachment for my '05 Swisstool and have it working almost perfect. I decided to see if it was a fluke or it would work on all Swisstools. Got out my '98 and found it fits but dosen't lock in like the '05. Upon closer inspection I found that the '98 pliar head is missing the groove that the back spring latches into on the '05. The '98 is just flat. Then while editing the attached picture ('98 on the left) for clairity I made another discovery that leades me to belive the "square" feature was initialy a happy accident that became an intended feature. As one opens the '98 there is a flat at both 90 degrees and full open that the spring rests on. With the '05 it's not a flat but a purposfully formed groove for the sping to catch in. After seeing this I decided to check out my Spirits also. Low and behold they have the groove for the spring to catch in while in the full open position just like the '05 Swisstool but are completely round across the 90 degree angle. No surprise as the ergonomics of the Spirit handle prevent it from use as a "square" even if some one wanted to. I would love some help from those with a larger collection of Swisstools in confirming and possibly dating the change I have come across or a link to further info on this if any one knows of such a link.