Help identifying master cylinder, 20mm from 1977????

Hi all, i am real confused about this one. A guy sold me what was supposed to be a 19mm master cylinder from a 911. A quick research led me to thinking that it wasnt the one for my 914. Can you help identifying what i have?
Rumours say it could be one of those rare 1977 20mm for use with «S» calipers.

Please go to:
http://www.farleydw.com/mc_1.jpg

Thanks.

the cylinder size should be embossed on the bottom.

[This message has been edited by Bill Verburg (edited 02-12-2001).]

Nothing rare about it ... looks just like a replacement for any power-boosted 911.

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Warren Hall
1973 911S Targa