I have a CT2 CT3 cylinder that was on a AT2MX that i have and it was just brought to my attention ( i was selling the cylinder) that there is what maybe a modification of a small machined or drilled hole from the inner cylinder wall in the exhaust port / outlet.

Was this a period correct modification when a CT cylinder was put on a AT?
What was this modification for?
Can the cylinder still safely be used as is as I am not using it but if I were to sell it I want to be sure that is a good part, and it would not cause any damage or unresolvable tuning issues.

thanks for your comments!

The holes are there from new. No real idea of the purpose.................

I've never seen the compression release hole on a new CT2/CT3 cylinder, but "normal" on the similar AG175 cylinder if I remember what other members have said.  (Mentioning this because you might need to identify if you have an AG or CT cylinder.  We didn't get the AG in the US)

That said, adding a compression release hole was something Yamaha wrote a service bulletin on for the US CT3 (and all CT models) that suffered premature kick start failure due to improper tuning or incorrect starting procedure.  They did not have a part number in the bulletin for a factory-drilled cylinder, just instructions on how to do it.  I've seen a lot of CT cylinders and have never come across one in the US that has been drilled.  (I think Enduronut posted he has one and called it very rare?) 

If the instructions are followed as to the precise location of the hole, Yamaha says the pressure in the cylinder at that point and the pressure in the exhaust port equalize when rpm is above about 1,000 so normal running or tuning is not affected if the hole is added.
 

Bought a nos CT cylinder from a former Yamaha dealer, still in the original box, and it has the hole in it.

1976-1978 Yamaha RD400 bikes all had the decompression passage in their cylinders to make them easier to kick over.

In NZ the only cylinders with holes are AG175. I used one on my AT1 'with base plate' "due to rod change from piston port to reed". They're everywhere down here those particular AG cylinders. Porting is strangled & made for reasonably sedate farm use. Nothing wrong with them, just not very sporty. Used the same cylinder on my RX125 with same result--went well, just not fire breathing.

Yamfan wrote: "Bought a nos CT cylinder from a former Yamaha dealer, still in the original box, and it has the hole in it. "

Can you post pictures of the cylinder and the box showing the full part number?  That would be interesting to see.

Box has long gone, but still have the cylinder. From the part number it was for a CT3

CMSNL now show the part number with a "-02" suffix, so presumably one of the changes was for the hole.