The little silver disc on the knob pry's out. Then you will see a small clip that comes off, and then you can remove the knob, and dial. Should be a small spring behind the dial. Then there are I think 2 bolts behind the dial that hold the timer in place.

This is the timer you need.  It is part number 204889 (2-4889).

Here is a NOS for less money.

That's a nice, versatile machine.  I think almost 50 years is an impressive run for its original timer.  The NOS (new old stock) timer linked above should be good for another 50 years.  An excellent deal at $1/year and well worth it for keeping such a faithful -- indeed dependable -- machine going.

This is wiring diagram for the A407.  It might be useful in hooking up the new timer. 

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