Steelhed, I can't give any direct help, but here are two sources for the info you need:

1. The Clymer repair manual folks have a book on rotary tillers that includes info on your old Gilson (same as some Montgomery Wards models). Perhaps a local library, or used-book store has one, or you can get a new one for about $20 at
http://www.clymer.com/Book.aspx?bid=834&title=Rotary+Tiller+Service+Manual

HOWEVER, I don't know for sure that this book will show you belt arrangement.

2. Gilson was sold to Lawn Boy. Some Lawn Boy repair shops (especially ones that have been around for a long time) still have Gilson manuals, and can get replacement parts. It's worth investigating.

I second what 12 gauge said you have Gem in that tiller the tiller advertisment I saw was from 1925 Gilson tiller Gilson started garden power tools they made own parts from castiron.

i need to find a belt for a model#5100342860 ser.1742688?

I have an owners manual and parts list for a Gilson 51134,
with no letter after it.
If you still need it I can fax or hopefully email it to you.
I just joined the forum today 4/17/12

I have a Gilson Plymouth 51111 and looking for info on the belt routing & pulleys.

Got these manuals scanned today. Hope they help those that have been looking for them. My dad got the copy of the 51134 by emailing Toro's customer service. The 51081 manual was with the paperwork of a working model 51134 tiller that was given to him.

Gilson Model 51081 Manual - https://docs.google.com/open?id=0B9STJ5GP1jRNc3JDbHk4Zm15TnM

Gilson Model 51134 - https://docs.google.com/open?id=0B9STJ5GP1jRNZjhQVjFQc0JpUWs

B&S Engine manual for models 100200 to 100292 and 130200 to 130292 - https://docs.google.com/open?id=0B9STJ5GP1jRNeFY0c0lyYkFnQ0U

I'm looking for a manual for the following item, manufactured by Gilson, a Plymouth Model # 51000 I'm assuming the 43165 is April 30, 1965, maybe a mistake in the stamping?? Serial # 2781924.

If anyone can steer me to an Owner's Manual or a Service Manual I would greatly appreciate it.

Thanks.

JayGee

Not sure if anyone gets updated if there's a new comment, but I just picked up a 51135 8hp rear tine Gilson and same deal, seems like someone messed with the drive belt and couldn't get it to work after that.

Can't find any info on it at all! Any help in finding a manual for it would be awesome. Pretty sure I'll be able to figure out the belt situation, will post a pick later, it is too cool.