Does anyone know the difference between these two Sako actions in a .375 H&H?

I think that the designations you are using are confusions between the previous "A" series actions and the current Model 75 action.

The "AV" was the "long" action of that series and the equivalent of the older L61R. It was a modified Mauser with two opposing bolt lugs and a 90-degree bolt lift.

The currently manufactured Model 75 has three locking lugs and a 60-degree bolt lift and comes in five action variations ranging from small to large: The I, II, III, IV, and V. The largest, and the one used for the .375 is, I believe the V. I think that a IV has an '06-sized bolt face, but I'm not certain of this.

At any rate, there was never a IV in the older "A" series. There was an A-I, A-II, A-III (also a long action in both standard and magnum bolt face), and an A-V. Why Sako skipped the IV designation, I can't say.

The AIV was the deep magazine action for the Sako Safari, mine held five .375H&H in the box plus one in the chamber. Same with .300 H&H (I had a .375 rebasrreled) but with the short magnums, i.e., .338, .300 Winchester, the cases are not tapered and only four would go in the box. The big .375 magazine capacity was welcome when I went to Africa in 1988, had to shoot a buffalo 4x and while waiting to see if he would get up again it was a comfort to still have two cartridges in the rifle.