Jeffer
Hello & Welcome to the Forum!

The Lionel No. 20 SG Crossover has been in production for years.
The brass plate is the part identification.plate.
Is yours all metal ot does it have plastic parts?
Early ones were all metal, later had plastic pieces or all plastic except the rails.
l see lots of them for sale online, you might be able to compare yours with posted pictures of items for sale in online auctions to date it.
There might be track collector groups online that might be able to help you date it.
l'll have to look at my SG#20 crossovers and see if l can find any dating clues

Happy Rails.
dave