They are not all the same.
Different engines use different OWB's, although some OWB's work on multiple engines.
If you have the old HPI "Big Block" .21 (3.5), the HPI part number for the OWB is 1430.
Go to eBay and search for HPI 1430.
If it looks the same as the one you got, then that's the one you need.
Ofna's 52093 is the same thing and it used to be cheaper than HPI.