Purchasing clutch master cylinder

[Brian DeFord <bdeford@erols.com>]

I went to order a clutch master cylinder today, but there was a strange
problem that came up: 

Two different manufacturers were listed.  One of them listed only one
master cylinder.  The other listed two, with one being an 11/16" and the
other being a 3/4" tandem.  What is the difference between the two and does
it matter which one I buy?

Thanks,

Brian