An S2 L67 is an S2 L67 to the computer. That likely is not your issue. There is a very common issue in these transmissions called the 4th gear clutch hub. The splines on them were not hardened until/after 2003. And because the trans is not easily interchangeable, your brothers car probably has it's original transmission.
On the first/second design and aluminum gasket or not, please see the below info.. He can have plastic and second design.
A question was asked and I figured I'd toss this out in the open as well for general info to all.
When ordering LIM gaskets on the 3800, there are two listings.
First design w/o locating pins
Second design with locating pins
Soooo what's a locating pin right? On the second design there are two rubber pins on the rubber end seals of the LIM gasket set. 96-97 year engine blocks are technically are the first design w/o pins. 98+ are second design blocks. The difference is not worthy of having two different sets of gaskets. IMO the second design should have superceded the first and only one version should be available as the pins and their function is minimal at best.
96-97 engine block. Look at the top and bottom of the picture at the block. See the solid line of the block from head to head. That's the first design w/o any holes for locating pins to sit into.
98+ engine block. Look at the left and right ends of the block. See the two holes on each end. These are where the locating pins sit into.
When putting second design gaskets on a 96-97 block the pins simply lay over the side and still do the job of holding the gasket in place before the LIM is put back on.