My impression was that it was a flat 50% penalty out of cover for the lmg42 (so the same penalty as their kars), which got reduced to a flat 25% penalty in cover (so 75% of a gren lmg42 in cover). Not based on any testing, just my impression from the wording.
You might be right but the patch notes indicated something else:
Chance Part 2.
"Captured slot items now suffer from a -50% accuracy penalty, and +300% accuracy bonus in cover no longer applies to them"
A flat penalty and no bonus.
V1.5.1 UPDATE
Slot weapons now benefit from partial application of the cover bonus: Out-of-cover: -50% accuracy; In-cover: -25% accuracy
FINAL
Cover bonus for slot items from 300% to -25%
In the final version there no mention of removing the universal 50% accuracy penalty from slot weapons. It would be clearer if they changed the cover bonus to x150% that would translate to 75% of the grenadier LMG.
Generally speaking it was major nerf (and justified up to point) to Osttrupen reserves (and any squad picking up weapons) who had x1.5 bonus in their LMG and now in the best case scenario their LMG has half that DPS, and have their slot reduces from2 to 1.