|How many times do I have to point out this stuff? From Ewbank's guide to the Blue Mountains (1967).
If a climb uses only one pitch (this is a typo, it should read piton. BA) for physical aid, the climb is graded free and the piton mentioned. If a climb uses two or any number of pitons for physical aid, but they are separated by free moves then the climb is still regarded as free with aid. For example: Pitch 3. 60’. (crux). Straight up the groove, ‘4 pitons for aid’. However if two aids are used in succession with no free climbing in between., then that particular section is regarded as mechanical. A climb, may therefore be free, aided, and mechanica1, though only the two grades are used - i.e. 18 and M.5. while the aided portion of the climb is described verbally in the description.
The easiest mechanical grade (M.1.) therefore applies to such things as two firm bolts, close together, in any easy position on good rock.
When it came to people slumping onto a piece of gear for a shake-out there was no way of indicating it in the overall grade, so M0 was added to the grades. It did not exist in the original Ewbank grades, OK?