Hi Roy,
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Summary: The correct one for a 125 is the Right Side (Without the spark plug in).
Notes:
It is possible that the previous person was attempting to get more performance out of the Lambretta and simply tried a different head.
The following You Tube video explains this in detail (See News Post or Shaft Riders post)
Click Here (Or search FINE TUNING THE CYLINDER PORTS DIO
If the previous owner was not very skilled, it may simply be a case that they put a new head on, in an attempt to gain more performance. It is also possible that the previous person may have skimmed / smoothed other areas out on the cylinder / piston and may the old head to achieve better performance, so these areas should be checked and then a decision made on if you want to put things back to the way they were, or leave as is. If skimming has taken place and the person has overdone it, this may cause performance problems, rather than enhance it.
I personally like my Lambretta in as original condition as possible, so do not tamper in these areas.
Other link for example of a head for sale (They come up on e-bay at a fraction of the cost ever so often)
Hopefully the one you have is all OK!