1979 Monaco 20 Cmes Struck On 10 Cmes Planchet?

I recently discovered this coin in the treasure chest of my LCS. At first glance I thought it to be PMD, but being on sale that day only for $4.99/lb, I put it in my bag anyway to examine it closer at home. (Ended up with 5.6 pounds!)

After examining it more closely at home, I believe it to be an off-planchet error!

A 20 Centimes struck on a 10 Centimes planchet!

I've never owned an off-Planchet error or actively pursued them so I don't know much about them.

Here's what I know:

This is a 1979 Monaco 20 Centimes that is very thin and is missing the right outer design elements on both the obverse and reverse of the coin.

It has an obvious planchet crack at 12 o'clock on the obverse and 6 o'clock on the reverse. (coin alignment)

It weighs exactly 3 grams on my scale (should weigh 4 grams) and is slightly irregular in shape but measures approximately 21 mm in diameter using my calipers.

It is non-magnetic and appears to be aluminum bronze.

It is the exact weight and composition of a 10 Centimes and its diameter is WNL.

So what do you guys think?

Did I just find my first off-planchet error coin?