I’ve looked at a few more photos of the Achilles, and I don’t see a clear case for a covered turret.
You can identify the Achilles 17Pdr, from other tank destroyers/self-propelled guns, by the large muzzle brake and the counter-weight just behind it.
The two photos in the OP could have been a field mod. The additional armour looks the same in both photos, but perhaps it’s photos of the same vehicle, or an Achilles field modded by the same workshop.
I did find another site/source that talks about the extra 17mm and 20mm plates, but they suggest the 20mm was just added to the front of the turret top, not covering the whole turret.
Here:
http://www.wwiiequipment.com/index.p...anks&Itemid=57
/Cross