Kubota L3830GST, B7500HST, BX2660. Formerly: Case 480F LL, David Brown 880UE
Looks like cast weights as has been mentioned. It appears that they also have a set of weight plates under the middle of the mower. See the area outlined in orange in the picture below.
When I worked for a lawn care company, we had liquid cooled 60" and 72" exmarks. The 60" machines had weight plates in the front middle because the deck was not heavy enough to counterbalance the weight of the engine in the back.
Weight kits are standard when you get a grass catcher. I am very unhappy with the poor design of the catcher I have on my HUSTLER mower and the inadequate amount of weight provided. I do however like the mower.