SP89 upgrades
I have two HK SP89's one is a "KA" date code and the other is a "KD" date code SP89.
-The KA SP89 has a bump on the left side of the receiver for what I assume is a bump stop for the selector lever in the safe position. The KD SP89 doesn't have this feature.
-The KD SP89 the rear lower section of the receiver, where the push pin goes. there is a small piece of metal spot welded for support. The KA SP89 doesn't have this feature, just welded.
-The KD SP89 has the newer "Action 3 bolt head".
-The KA SP89 markings are different, "Made in W. Germany". The KD SP89 is marked just "Made in Germany".
These are the differences I can visually see, I haven't disassemble the bolt and remove the rollers to see if there are any other upgrades inside the bolt head.
Hope this information helps you out.