Easy way to tell is to look at the model designation on the inside of the crane. IIRC, all generation 3 and above M38s can handle +P loads.
My M38, purchased in 1996, is marked "M38-2". This is a second generation M38 and therefore it cannot take +P loads (trust me on this one, I managed to shoot my revolver out of spec).
Justin
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Justin T. Huang, Esq.
late of Kennett Square, Pennsylvania