difference between a 742 and a 489?

I found some rear axle info that suggests the lowly 741 was used for the Max Wedge,, link supplied,

64 Max Wedge 53-1/4"

Parts Manuals list four different axles for this housing:
2070573 - 741 case w/o SG 30-1/2" overall;
2404003 - 742 case w/o SG 30-1/2" overall;
2070695 - 741 case SG 30-7/16" overall;
2404050 - 742 case SG 30-7/16" overall;

http://www.bigblockdart.com/techpages/875axle.shtml

and I've seen it other places as well..

hope it helps