Engine code in a 1969 Firebird? XH? WQ?

[kc5swi]

I have a 69 firebird Trans Am, ra3, m-20.. It appears original unrestored in every way, down to factory hose clamps, etc..

The trans is a M-20 with matching vin number stamped in the case. The engine is casting 9799914, HO49 cast date.
Car built 10-1969. Everything appears correct but the engine is a XH code, which from the internet should be from a Grand Prix. I know this car was built very late in 69, it has the formula style factory steering wheel. I see several late built firebirds have the same block casting code. But what about the XH? Could the factory have made an error? It seems unlikely the original motor disappeared and a date coded correct block from a different car model of the same year appeared? Anyone seen anything like this or have thoughts to share? The block didnt have a vin number stamped on it.
Thanks!

[Duane Stumpp]

XH is an automatic GP 400. Missing VIN is a clue. Perhaps original engine was destroyed under warranty and replaced with this block rather than an SR version.