Irregular diameter
One unusual observation is that coins minted at Hyderabad are usually slightly larger than coins from other mints. I have examined 62 coins, of which 16 are from Hyderabad and 46 are from Kolkata, Mumbai and Ottawa. On an average, the Hyderabad coins have a diameter of 19.23 mm, while the average for the other coins is 18.99 mm. This difference is sufficient for the non-Hyderabad coins to drop out if you hold a stack of coins between two fingers.
Korean blanks
Some mid 1980ies coins have a distinctly flat edge, as opposed to the normal, somewhat rounded, edge. The explanation given in the SCWC 2007 (p. 993) is that the blanks resulting in the flat edge coins are Korean-made, as opposed to the usual Indian blanks. The flat edge phenomenon is to be encountered on coins of 1984-1986.
Flat edge (= Korean blank) above and rounded edge (= local blank) below on two different 25 paise coins (click image to enlarge).
In the case of the Korean blanks, the edge reeding is not very deep, and sometimes appear faint. In some cases the denticles along the rim on obverse and reverse are not fully struck up. This appears to be the outcome of poor dimensioning of the blanks.
No coins produced from Korean blanks earlier than 1984 have been recorded. Apparently, the supply of blanks from Korea was part of an attempt to alleviate the coinage crisis of the 1980ies, when the production of small change couldn't keep up with demand. As noted earlier, the mints of Ottawa, Birmingham, London and Taegu (in South Korea) were supplying coins for India during 1985-88.
Six coins minted from Korean blanks are listed below, together with three regular coins for comparison. First the edges.
Note coin no. 3, 7 and 9 from above - the coins minted from local blanks. These are clearly the odd ones in this group. The others are minted from Korean blanks. Note the faint edge reeding on some of them. (Click image to enlarge).
Now, the same coins in the same order as in the image above (click images to enlarge):
Wide D 3 1984 Kolkata, Korean blank.
Wide D 3 1984 Mumbai, Korean blank.
Wide D 3 1984 Mumbai, local blank.
Wide D 3 1985 Kolkata, Korean blank. Note the faint denticles along the edge on obverse and reverse.
Wide D 3 1986 Kolkata, Korean blank. Note the faint denticles along the edge on obverse and reverse.
Wide D 5 1986 Mumbai, Korean blank.
Wide D 5 1986 Mumbai, local blank.
Wide D 5 1987 Mumbai, Korean blank. Note the faint denticles along the edge on obverse.
Wide D 5 1987 Mumbai, local blank.
Apart from the coins illustrated above, I have also seen:
wide D 5 1986 Hyderabad local blank
wide D 5 1988 Mumbai local blank
wide D 5 1989 Mumbai local blank
Note that 25 paise coins minted from Korean blanks are encountered among coins from 1984-87, and subtypes wide D 3 and wide D 5 (corresponding to KM# 49.1 and 49.5) are both represented. Korean blanks were used at the Kolkata and Mumbai mints, but apparently not at Hyderabad.
