This looks like a group 53 battery. VARTA made in Strousberg Deutshland. It is NOTa VRB meaning sealed battery but it is indeed a full-on calcium/calcium plate battery. Minimum gassing corrosion and water loss. The Germans are famous for in-car battery locating the old beetle with a battery under the rear seat (group 42) is a perfect exanple.
A 53 is a large battery. Bigger than a group 65 or 29-30-31
Because of its location I most strongly urge you to strictly stay with a calcium calcium battery and definitely NOT replace it with a standard battery. An AGM 53 would be fine. Group 53's do not grow on trees. It is a hard-to-find group size while on the road.
Many of the older Bosch batteries had cell plugs. Twist hard enough and they'd pop out - for good. Not threaded.
If original that battery is nine years of age. If diesel you will need ONE GAUGE truck jumper cables to jump start. Far larger than all but the most expensive jumper cables.
Delco USED TO HAVE a calcium-calcium 53 replacement.
But a pre emptive repacement is up to you. Now you are aware and now you have the info to make a decision in your best interest.