Yashica T3 light leak

[alter ego 6x9]

Sometimes there is, sometimes there isn't. When there is, it is always in the same place of the frame (and same shape). I inspected carefully the inside - no damages or deterioration of light seals, all very clean... So - what could be the reason?
Below are two photos taken in the sequence - one shows the leak, another is without.

 

[E. von Hoegh]

Is there something in the mirror box causing a reflection? Remove the lens, set the shutter to "T" or "B" so the mirror is up, see if some black paint is missing somewhere.

 

[gone]

Could be the film pressure plate too. I'd ck that carefully as well.

 

[alter ego 6x9]

Could be the film pressure plate too. I'd ck that carefully as well.

Right, based on the location of the leak it must be it... Thanks!