Hi,
The first two digits in the code number of your engine are the year the engine was built.
The next two digits are the month and the next two the day the engine was built.
So your engine was manufactured 23 April 1990.

Provided you engine is the original for the mower, then you mower was likely to have been manufactured in late 1990.

The Identification Label could be at the back of the mower.
I currently have a 1996 model Murray that I'm working on.
Here is a photo of the rear of that one and the ID label showing the year of manufacture and model number.

[Linked Image from i295.photobucket.com]

[Linked Image from i295.photobucket.com]

Hope this helps.
Regards
Murray



Cheers, Murray