Sorry to have put you through that scraping and brushing to find the same numbers!
I'm going to venture an educated guess on this engine.
Here's some facts to somewhat justify this guess.
In 1970, the federal government issued regulations to begin in 1972 model year, requiring all engines, frames and transmissions to be stamped with a partial VIN number for vehicle theft identification.
Sometime in the early 1971 model production year, GM voluntarily began their Engine Partial VIN Number and Engine Assembly Code Number stamping on assembly of an engine into a vehicle, and in the process, standardized the stamping location and number format, as mentioned and displayed in the above message.
This numbering format was formally in place and standardized in all GM assembly plants by the beginning of the 1972 production year.
So, now what about your engine?
Here's my guess;
From your detective work with the pan off and the discovery it's a 4 bolt main, and the unusual stamping sequence on the passenger front pad, I think this is a 1969 - 1970, 4 bolt main 350 cid engine.
The 302 cid engine that was made from this block casting in 1969 (only) was 2 bolt main, the 1969 - 1979 350 cid was 2 & 4 bolt main, 4 bolt being most common.
Without a decodeable Engine Assembly Code, it's going to be tough to figure out what was the original engine configuration, horsepower and vehicle it was installed in.
If in fact the windage tray is GM, it is possible this engine came out of the factory in your Camaro.
The addition of a windage tray was used on high performance engines/vehicles.
It's also possible the windage tray is from a later model engine and added by a previous owner.
The vintage of the engine and the Camaro are very similar.
But, this many years later and several or many owners later, it is going to be hard to tell!
Another 'test', would be if you ever have the heads off, measure the bore, a stock 350 is 4.001", that'll take a dial caliper or ID micrometer to check.
At this point in time, it's a safe guess the engine may have been bored as well, +0.020", 0.040", 0.060" are the common overbore sizes.
I hope some of this information helps!
Good luck with your build and don't be a stranger on the forum.
Stop in from time to time and tell us what's happening with your project!