It was made by Roper. I also have been unable to find an owners or repair manual for mine, or any other information on it as well. I bought mine back in the '70s so it may be just too old to get info on.
sears.com has all the manuels you would need. just input your model # and it will give you a pdf to download
I did not find a manual but it seems that vendor number belongs to Husqvarna.
http://www.searspartsdirect.com/partsdirect/index.action
http://www.sears.com/shc/s/nb_10153_12605_NB_CSHome
you can find a users manual or troubleshooting guide for the Homelite rototiller model HTC 12, on the Homelite website. Click on the Service and Support tab and then on Operators Manuals.
Google, plain out and simple.
model 5 C Ser # D001943
Looking for the manual for Craftsman Yard Pro Rear Tine Tiller model 130202 Type 312901 Code 87120107
eBay has a number of these up for auction.
You can find a manual for your Rally rear tine self perpelled Rototiller model RYX820K, manufactured by American Yard machines, on the American Yard machines website. You will find manuals for all of their products on the website.
call Sears warehouse and you can order one. Also check online as they may have it there.
Check the owners manual on the web for your product, for example the F200 series specifies the same oil as the engine SAE30