I was noticing that the erasers in these are really small. Pentel’s 1979 catalog says that the Z2-1 eraser fits all pencils except for the PD345, PS1042, and PS1045. But, they are silent on which erasers would fit. Does anyone know?
Hmm interesting question. Lucky for you I have a PS1045 here with me on my desk. Unlucky for you my calipers are downstairs in the shop. 
I don’t know the answer to your question but I grabbed some random pieces within reach to see which erasers fit.
In the picture below, the erasers in the pencils on the left do not fit in the PS1045. The erasers in the pencils on the right do fit.
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PS1043 on top and P1075 below. Both are 21mm long but the smaller one has a 2.1mm diameter vs 4.1mm.
Measuring things again both the PS104x pencils have 3mm erasers.
Looks like I mixed up the PS1045 and the P1075. 
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Pretty sexy testing lineup on the desk 
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