By 1976, the Mumbai and Hyderabad mints were both minting two subtypes, both with the wide D (see this entry). These applied Ashoka sculpture version d (for "wide D 3", common type), and Ashoka sculpture version e (for "wide D 4", scarce type). As noted earlier, the Kolkata mint ceased producing the narrow D types in 1976. In 1977, the Kolkata mint began issuing the "wide D 3" type of the Mumbai and Hyderabad mints.
A 25 paise coin of the "wide D 3" subtype, minted at Kolkata in 1977 (click image to enlarge).
The last recorded minting of the "wide D 4" subtype (minted by the Mumbai and Hyderabad mints) is a 1981 coin. The "wide D 3" subtype (now minted by all three mints) was issued simultaneously and beyond.
Remarkably, according to the listings in the SCWC 2007, the Mumbai mint temporarily resumed minting the former Kolkata subtype "narrow D 1" during 1977-79. (Note added Sep 3 '07: this was indeed the case).
Thus, during 1977-81, the minting of the narrow D types was discontinued at the Kolkata mint, in favour of the "wide D 3" subtype, introduced earlier on by the Mumbai and Hyderabad mints. The minting of this subtype continued throughout 1977-81 and beyond, while the minting of the "wide D 4" subtype ceased in 1981. Apparently, the Mumbai mint temporarily resumed minting the former Kolkata subtype "narrow D 1" during 1977-79.
