Just to add to the above, looks like you are not doing much mileage, the battery may be draining down; indeed a battery of that age may be within seeing distance of its demise!
When you start, the alternator has to put out a large amperage, which in turn places more mechanical load on the belts and pulleys causing slip. As the battery recovers, the load diminishes and squealing stops.
As a matter of interest, Regarding part numbers: 6PK1203...the 6 is the number of ribs and the 1203 is the circumferential length of the belt in mm.
