1M99 was the 12.45 to Liverpool, the old 'North Mail'. The dmu may be the following 13.25 service to Truro or just perhaps a St Ives Branch train?

Peter Kerslake says "This will doubtless be the Royal Mail travelling post office train and its supporting coaches leading the formation of the 'North Mail' seen here at Penzance, both working through to Bristol Temple Meads together with D868 'Zephyr' which entered service at Laira in May of 1961.
A Westcountry locomotive for its working life 'Zephyr' spent time housed at Laira and Newton Abbot before ending its days from Newton Abbot in October of 1971."