The first stamp is a 10c Rivadavia Red Type II with inverted M.G. overprint. The stamp is used, and appears genuine. Use departmental officials are considerably more desirable than mint ones. Because mint ones are most likely reminders-these stamps were not sold to collectors-the used ones are the desirable ones.
Here is a detail after a 180 degree rotation.
The second specimen is a 50cNGR-SO.
This stamp has ink from the sheet below it, which means that there are at least two sheets of 100 stamps with this variety, quite likely more. The stamp is mint and appears genuine.
Here is a detail of the overprint.
Here is a detail of the back-printed overprint after a horizontal flip. These two overprints need to match exactly since the sheets were stacked evenly on each other.