I had read about the issues where pre-2002 HK SL8-1 had an issue where the partial lug design of the SL8-1 bolt head hit the bolt stop catch off center, causing eventual cracking of the firecontrol group housing in the buttstock below the bolt stop. Apparently the G36 bolt catch was designed for a full lug bolt head, and the off center impact caused the polymer to crack. HK sent out replacement redesigned and beefier bolt catches for this issue for rifles under warranty, post 2002 models had the new part from production.
When I bought my SL8-1 used from the local gun shop, I asked them if it had this issue, they looked at me funny, and said NO.
When I took it home to clean it, took it apart, THERE IT WAS, a big ol crack, some 2 inches long that could be easily spread apart with light pressure. Even though I had planned on evetually putting on the TopNotch stock kit, I knew this would take months and I did not want to shoot it like this, the cracking could eventually split the entire FCG housing. $850 for a rifle with a fractured FCG housing was too damn much for me so I took it back, explained, and they were very good about it.
They let me get store credit, since it was a used gun, so I used the store credit and bought a brand new Heckler & Koch USP9 SD (plus refund of difference). I absolutely love the pistol. I was sad about the SL8, but decided it was going to cost too much in the end, with the crack. Too bad, the gas piston system is so simple and absolutley ingenious.
I suppose all is well that ends well, but if any of your are looking at buying a pre-2002 SL8-1, look for this cracking issue. You cannot see it unless you take off the stock, it is a crack right under or near the bolt stop catch, you can see it on the front of the FCG housing in the polymer if you attempt the spread the sides of the FCG housing. If you buy an NIB pre-2002 SL8-1, call HK to see if you have the retrofitted bolt stop on it or need them to send it to you.
Even if you plan to do the "G36" conversion, still an issue to be aware of until you get the conversion parts.
Note that this issue was caused by HK having to comply with the stupid 1989 import ban, forcing HK to change to a single stack mag and change the bolt head design for US import only. The G36 and European SL8 version, the SL8-4, does not have this issue as it has the G36 bolt head and the double stack mag well that accepts G36 mags.
Damn our government's stupid, petty firearms laws! They don't prevent crime, they just complicate things beyond belief.