In addition to checking the manual for acceptable part numbers, you might give Lycoming a call and get a surprise. When we were rebuilding Louise’s O-360, we found that the part numbers on the rods that came out of the factory-built engine didn’t match the parts manual for either original equipment or “acceptable replacements. this affected the rod bolt choice, and we scratched out heads for a morning, calling several engine shop owners that we know. Finally, we called Lycoming, and got an engineer on the phone within a minute. He said he needed an hour, and he’d get me an answer after lunch.
Sure enough, we went to lunch, came back, and we had an email with a Service Instruction that specifically called out a small (single day’s run back in 1990?) number of engines, by serial number, for which an alternate part number for the rods were approved. Ours was in that batch, and the specially approved rods were what was in the engine!
My guess? They were busy cranking out engines one day, and ran out of the normal roads. Engineering approved the use of these substitutes, and had to write an SI to cover the engines built that day. The engineer I talked with was polite and happy to help - and didn’t seem to know (or care) who I was, or that I was a journalist…just wanted to help a customer with a question.
So….give Lycoming a call!