The OP20 Upgrade to Zerk is a great choice for customers who prefer not to have an already included front tire with side tires and possibly. want a capstone. With this upgrade you will instead receive a beautiful tombstone on the front of the grave monument. The gravestone is made of 6 cm thick natural stone and is available in the same natural stone types as the grave monuments themselves. This gives a uniform look to the entire memorial. Upgrading to Zerk is an excellent way to add a unique touch to the grave monument of a loved one. Available with and without planting opening.