Singapore 50 Cent Coins years 1974, 1981, 1982, 1983 Collectible Vintage Currency Coin Mint Print Error

Regardless which year they were minted, these 7 coins have one thing in common which is, they are all having a mint print coin errors.

Notice the rim thickness are inconsistent where some edges are thinner than the rest of the coin. According to Susan Headley of Spruce Crafts:

"Many people mistake double-struck coins for doubled die coins. The critical difference is that double-struck coins are struck more than once while the coin is in the coining chamber of the coining press. When this happens, the first impression is flattened or sometimes obliterated by the second strike. Subsequent strikes will also flatten or obliterate the design from earlier strikes.

Coin dies that are not appropriately made result in doubled die coins. As indicated above, it takes several impressions from the coin hub to make a coin die. If the mint employee does not align the coin hub perfectly above the coin die, a second impression will result in a slightly off-centre coin die from the first. If this coin die is used to make coins, all coins made from the die will have this doubling effect."

Can this explain why there are 4 pcs of 1981 coins having this effect?

There are altogether 7 coins which are:

1 x 1974,
4 x 1981,
1 x 1982,
1 x 1983,

These coins come in a plastic bag suitable for storing coins.

Singapore 50 Cent Coins years 1974, 1981, 1982, 1983 Collectible Vintage Currency Coin Mint Print Error