old briggs twin cylinder flathead, this a 16 or 18hp? (pix)

I have an old briggs twin that was on a grasshopper ZTR mower I'm 'parting'. but I'm not sure if it's a 16hp or an 18hp? would some kind soul please take a look at the pictures on these 2 pages:
http://machines.50webs.com/Grasshopper_ZTR/index4.html (it's the bottom 15 images, last three rows)
http://machines.50webs.com/Grasshopper_ZTR/index5.html (it's the uppermost 5 images on this page)
and tell me what they think it is? alternately, is there something I can -measure- or 'look at' on the engine that'd tell me 'fer sure' which hp engine it is?
thanks a real lot,
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