Yep...I have drawings and parts list for all of 'em.
W-C 51 series toilets are the Imperial or the smaller version of it called the Junior. They were discontinued years ago...so except for a rebuild kits, very few if any parts are still available for 'em...and I'm not even sure about rebuild kits for this toilet. Thetford Norcold discontinued the entire W-C product line at the end of '08, so very few parts are still available for ANY W-C toilets any more...and they'll become even more scarce as what little is left on the shelf is sold.
Send me an email (to do that, click on my name above my photo at the left of this post) and I'll send both pages to you. It must be an email, not a PM...'cuz there's no way to attach anything to a PM.