Here is a part of the diagram:
Example d), as I’m sure you can easily figure out, is a heater. With all other things being equal, this one shows the inward arrows adding thermal energy to the hydraulic oil. I have left out the coolant flow arrows, which is common, meaning the coolant circuit isn’t specified.
Symbol is clearly a heater showing heating lines going in and out. Reference shows cooler coolant lines, which they were left off on heater symbol.
Missing horizontal line, but it follows format of filter.
From CHAPTER 4: ISO Symbols: