It (the holey con rod) came from the factory that way! Some time around 72 they were modified with the same part number.
When I pulled apart the engine which came with my Herald I found 1 con rods with hole and 3 with out holes.
Apparently your engine was also rebuilt sometime in history and a new con rod was needed so they used the later version.
Here is the answer I got from John Kipping regarding the holey rod: (originally discussed on the Yahoo Herald forum)
<span style="font-weight: bold">Sorry for late reply, shouldn't be a problem to reuse a mixed set as in they are all the same part number. If you were really keen weighing would be good but not really necessary.
John Kipping.
Yisrael Sundick wrote:
> Engine has definitely been rebuilt since it was
> rebored. There are also a selection of position #s on
> most of the conrods. Looks like this engine was built
> up out of parts from the scrap pile.
> The "holed" conrod was on the 4th cyl.
> As I understand, it should be the same weight, and
> that I should just reuse in the same position.
> Thanks
> Yisrael
>
>
> --- John Kipping wrote:
>
>
>> It shows someone has rebuilt the engine as the con
>> rods with holes in are a later (post 72?) variant but with the same
>> part number. Hence they are all interchangeable. All the original rods have
>> the position number etched on, if I was a betting man I would put money
>> on number 3 being missing. the number is on a ground bit of the
>> outside of the big end part.
>>
>> John Kipping, with broken hand - making typing even
>> slower than usual.
>>
>>
>>
>> yisrael_sundick wrote:
>>
>>
>>> Hi
>>> Why does one of my con rods have a hole drilled in
>>>
>> it near the big
>>
>>> end? The other 3 all are holeless. Was this the way
>>>
>> that they were
>>
>>> balanced? The hole goes all the way through the
>>>
>> rod.
>>> Is this acceptable, or has it been exessively
>>>
>> weakend? There are no
>>
>>> signs that it is bent, and the bearings had even
>>>
>> wear indicating that
>>
>>> it is not twisted.
>>> Yisrael
>>>
>>>
>>>
>>> </span>