I think all XJs came with brass heater cores. My 2000 XJ had one. Would be expected for other new Jeep models use an aluminum heater core. I don't think any OEM makes heater cores out of brass anymore.

I put a brass heater core in my XJ as well. It was made to TSMC which I suspect is the only company making brass heater cores for the XJ. I found that when I attempted to use even very thin foam for the seal around the heater core that I couldn't easily close the HVAC box, so I ended up using thin foam around the core in only a few locations. No problem getting plenty of hot air.

What I wish I had done was spend some time insulating the hot air portion of the HVAC box to reduce heat transfer to the cold air flow path and to the exterior of the box. If the outside air temperature is higher than perhaps 65F, warm air comes out of the vents even with the mixing flap set to cold. And I have verified that the stepper motor controlling the flap is functioning properly. The new core seems to get much hotter than the old one (perhaps no surprise), and it seems to radiate heat outside the HVAC box and to the cold air flow.