The production dates I have for the P30CH (from Marre, "The Contemporary Diesel spotter's Guide," Kalmbach edition of some years back) are 8/75 to 1/76. It was thus introduced after any other (domestic) U-series model, but well before the introduction of the Dash-7 (same source says C30-7 production started in 9/76, with the first B23-7 in 9/77).
Early Dash-7 were an evolutionary development of late U-series models with relatively few technology changes, and the carbody of the P30CH is obviously different (probably involving at least some changes to equipment layout inside), but it seems safest to bet that the "Pooches" were more like U-boats than like Dash-7 in any way the two differ.