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[stepho]

Here is a summary of the 20R and 22R and the differences between them.
http://members.iinet.net.au/~stepho/eng20r.htm

Quite often the ID number (which should start with 20R or 22R) can be cleaned up with a bit of industrial cleaner.
Or rub it with some fuel soaked cloth (to remove grease) and then some sandpaper (the flat metal becomes shiny while the dark grit stays embedded in the numbers).

Luckily, both engines will fit your '73 Celica.

- Stepho