you mean with the letter suffix in the stock and the odd looking eagle stamp? For me I haven't seen a suffix stamped on the handguard/stock of German K98 rifles. The bird stamp looks like it was restamped but was a light stamp and it doesn't quite look like the typical bird.
Looks like the original number was 353 and someone added the other digit between the 3 and the 5, using a slightly smaller number stamp for the 6/8. The 6/8 is definitely shorter and narrower than the 3 or the 5. Bottom loop should be the same size as the bottom loops in the 3 and 5 in most type fonts (as is clearly shown on the receiver).