Ok, I wrote some of this info so many times that I've decided to write a page where I could point users to read it directly...
Here are the SourceSafe 6.0 versions available:
VSS 6.0 RTM - This build is what shipped back in 1998 in the Visual Studio 6.0 box and as standalone product. It's a combination of binaries from builds 8169 and 8163.
The VSS 6.0 build numbers are 4-digits numbers equal to (8000 + number of days since beginning of 1998). E.g. 8001 is 1/1/1998 and 8169 is 6/18/1998.
VSS 6.0 builds before 6.0b - There are a couple of builds shipped between 1998 and 2001 on which I was not able to get too much information.
8383 - Shipped with Visual Studio 6.0 SP3 (some flavor of original 6.0)
8790 - SourceSafe 6.0a (my guess is that it shipped as standalone product)
8835 - Shipped with Visual Studio 6.0 SP4 (some flavor of 6.0a)
8987 - Shipped with Visual Studio 6.0 SP5 (some flavor of 6.0a)
VSS 6.0b - This build shipped in 2001, but I don't remember the occasion; my guess is that it shipped with a beta version of VS2002. It's a combination of binaries from builds 9119 and 8163.
VSS 6.0c - This build shipped in 2002 in Visual Studio .NET 2002 Enterprise box (VS7.0) and as standalone product. It's a combination of binaries from builds 9447 and 9350. Later, Microsoft included the same build into Visual SourceSafe 6 Service Pack 6 (VSS6SP6). Users of the older builds can upgrade for free to VSS 6.0c by installing VSS6SP6 (a 5.2MB download) from this MSDN page. Users of VS2002 and later are strongly advised to upgrade at least to this build (older builds don't recognize all VS2002 project types, the source control integration experience is worse, there are also 4 years worth of bug fixes)
VSS 6.0d build 9848 - This build shipped in 2003 in Visual Studio .NET 2003 Enterprise box (VS7.1) and as standalone product. It's the original VSS 6.0d release. 6.0d builds address various security issues with older builds, are built using VC7 instead of VC5, etc.
VSS 6.0d build 31222 - This later VSS 6.0d build was included in Visual Studio 6.0 Service Pack 6 (VS6SP6). Do not confuse this with VSS6SP6 that contained VSS 6.0c! Users of older builds can upgrade to this VSS 6.0d build by installing VS6SP6 (a 62MB download) from this MSDN page. You can upgrade your SourceSafe install for free to this build even if you already have VSS 6.0d 9848, and even if you don't have VS6 installed on the machine. You'll notice the build numbering scheme has changed from the incremental 4-digits numbers to 5-digits numbers representing year-month-day when the build was created.
Of course, there are newer VSS 6.0d builds available (as of writing this post, 61101 is the last build) that are addressing various specific problems some customers encountered. If you think you have a problem one of these builds may fix you can obtain them by calling product support (or request a QFE and have a new build created if the QFE is approved).