The Monticellos are known for handle issues. The large "barrel" ( identified as 'hub and elbow') unscrews to remove the handle and access the cartridge if necessary. They are put on hand tight, but late in life may need a strap wrench to loosen. Notice on your parts diagram there are different versions of the handle adapters etc but if you get all new that will be best. Moen ( 1-800-buymoen) may send you parts if you describe yourself as the original owner.