Can somebody who owns a Mirror with both mast step positions tell me what the measurement is from the forward (solo) mast step to the daggerboard slot.
I had a quick look with a tape measure on our ~1971 boat (25583) as the light faded yesterday
Re-reading your message I realised I've measured slightly different things but hopefully this is useful to you:
The distance from the outside front of the centreboard case to the bulkhead is 182mm.
The distance from the bulkhead to the rear of the rear (normal) mast step is 20mm
The centres of the mast steps are separated by 254mm
I forgot to measure the mast step block itself but this is around 50-60mm square
The forward shroud plates are ~245mm forward of the normal plates.
Do say if you could use more clarification - I can always go back to where she is kept and re-measure
I have heard that Mirrors sailed main-only & with the mast forward are difficult to tack. I discussed this with friends and it probably translates as 'suffers from lee helm', implying the forward mast position as built may actually be too far forward ie the centre of pressure is too far forward relative to the keel, so it might be worth being prepared to experiment with the forward step position in any case
hope this helps