Although it is always very difficult to judge overprints just looking at the relatively low resolution photographs, I am quite convinced that these are forgeries. Very well made ones, but fakes never the less.
The simplest giveaway is Soviet cancellation (easily recognizable on 5kop, 10kop stamps and may be some others). The only occupation stamp known to have Soviet Union CTO cancels was Raseiniai. Even on Raseiniai issues a different cancel was used. The best stamp in the set is 2kop issue with Alsedziai cancel, but even this cancellation is forged. The fake cancellation on this stamp is of quite high quality.
Same holds for the overprints. Excellent quality forgeries. I don't have these in my collection of forgeries, thus would love to see higher resolution scans (if you can provide)
In one of the other posts in this topic I have provided a scan of the true genuine overprint. Take a look.
I hope this helps. If you have further questions, Tom, please don't hesitate to ask.