Hi,Welcome
This is the forum for the predecessor to the Duramax.
Don't drive yours anymore,if the bolt head is gone the heater could fall out at any moment.And when it does you will lose all your coolant in seconds.
Go buy a new block heater and antifreeze with a different vehicle.
Then replace the old one,if you are lucky you will be able to dig out the old retainer,if you can't it wouldn't really matter because there are thousands of vehicles driving around where it's been left in after a block heater replacement.
90 Chev 3500 c/c 4x4,6.2na,400 auto,4:10 gears.DSG Timing gears,main girdle, isspro tach, pyro,boost,oil and trany temp.Dual Tstats, High volume peninsular pump,on shelf, Custom turbo and intercooler 85%complete. Change of plans for the dually, it's going to get a Cummins. Both trucks are Blue 90 4x4 crews