you can see the valve in this post, next to the sink, where I was replacing all the plumbing.
http://www.airforums.com/forums/240389-post6.html
I had to replace the shower valve due to corrosion, which made the interior too rough for the replacement O-rings. The fit is very tight and you have to install the 3/4" wood block (actually, plywood) between the valve and the fiberglass in order to get the right depth for the handle.
Be sure you buy a "shower only" valve or you will have a short descending length of pipe intended for a tub that cannot be drained and you'll have freeze damage eventually.
Zep