I have one of those old sprinklers too. It says Thompson on it, and it
is not all enclosed like the Nelson.

The repair parts you refer to are for much newer sprinklers. The
relics that Mel and I have are many decades old, I'll bet. Mine looks
like eBay #7763298758.
http://cgi.ebay.com/Vintage-Thompson...QQcmdZViewItem

I need the gear that he talks about and whatever assortment of seals
are necessary to keep the thing from leaking so badly.