The 2050045u is a code for telling specifics about the machine, the rest (418397) is the tractor serial number. That number puts your machine at 1972, being after 417643 but before 495749. For reference on serial numbers I use this website:
http://www.tractordata.com/lawn-trac...cadet-128.html
scroll down to near the bottom of the page and there's a serial number table. Hope this helps
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'70 107 with k301 engine swap
'71 106 with 38" deck
'70 147
R with factory replacement k321, 42" deck
'61 Original with 38" timed deck
'63 70 "pinkie"
1863 with 54" deck
'46 Farmall H, '50 Farmall Cub
105 x2 (parts)