Del Mar casement window operator part #21550

I am looking for a replacement operator for a Del Mar operator arm #21550. I spoke with "Jody" in customer service who sent me a Truth Dyad operator #39-316. It is the same type of operator that I need except the difference is in that the original part has a 4 inch arm that attaches to the window and the truth dyad replacement part is 5 inches. There are four windows that are identical and one of the other windows has a Truth Dyad replacement operator already installed. The problem is that the Truth Dyad operator that was previously installed does not have a part number on it and only has the Truth logo embossed on the base and arm.

Would you please see if you can get the appropriate replacement part for the Del Mar operator #21550

Thanks,
Joe