I bought what was stated to be an 1978 dt400 cylinder on ebay but I received what I believe to be a yz400 cylinder. It was no bolt hole for the clutch cable bracket and the ports are a lot bigger that a dt400 ports. I contacted the seller and this is what he said 

Your probably right & he's probably right too about selling it for more if you return it. If you had the high comp head it'd sure get your DT going well, but where do you stop as you'd need a better exhaust etc to get the full benefit. Pic won't expand up on my geriatric computer but no worries.

There should be a casting number in it someplace... often around the reed valve area or base of cylinder.

Check reed valve bolt pattern size against yours as well.

I looked on ebay and if I found the right one seller says it was "ported":...  if true, that would explain the larger ports.

Also the YZ would not have provision for the DT compression release.  The one in the auction clearly does.

ebay link to sold cylinder

If it's not a 77-78 cylinder, it's possible it's a 75-76 DT400 cylinder which will bolt on to cases but has a different reed valve. 

1.5mm over. I see Wiseco selling a 2mm over kit if its a live add. eBay item number:153060903822. Guessing it would be compatible with a ported DT cylinder. Or here's a 'Vintco' 1.5mm over kit if the bore is ok as is & save reboring.
eBay item number:183829400485. Gets expensive in the end & ideally need more compression too--to get the benefit of the porting.