Tractordata.com says this lawn tractor was made from eary 1980's thru 2000.
Here is a link to a discussion on the topic elsewhere
http://www.tractorbynet.com/forums/kubota-lawn-garden/280043-blade-removal-g1800-need-help.html
the parts manual of similar model is available free here at Kubotabooks.com go to mowers then to G1800. Better yet though is an up to date parts listing w/supersede numbers available online at kubota.com
Parts break down with drawings and part numbers
. Go here
http://www.kubota.com/part/parts.aspx
. Hit the [Illustrated Parts list] button
. accept the legal thingamabob
. Hit [OPEN]
. type in [G1900 ]
. Select your model
. Open the various sections to search out which one will show your the parts break down and part numbers for blades etc.
Manuals are at
http://www.colemanequip.com/, messicks.com, ebay, Amazon etc.