Isle Of Man Coin Error?

New to Numista so bear with me...I am researching what I think is an error on a low value 10 pence coin from the IOM dated either 1976 or 1977. The reason for not being able to give a specific date is the coin is one of five, all in MU condition, encased in a clear acrylic paperweight, popular in the 70's, all coins showing only the Reverse of the same. The 10 pence coin is the upper most and the detail of such very clear. I have att. picture of the paperweight and coin in question. There is a large round 'ball' shape next to the knee of the Triskeles under the word 'OF' and very small 'dot' above the letter 'L' of ISLE. Contacted the Pobjoy Mint in the IOM who reported no errors as such on this coin but could not give any rational explanation other than"not convinced it was an original coin". Further research revealed the paperweight was in fact made in the IOM.
It would be interesting if anyone could assist and advise on any further approach I could make.
In my opinion the coin, to me, appears quite genuine and is not the result of a deliberate effort to alter it after leaving the mint.

Could it be a bubble in the material surrounding the coin?

Thanks Oklahoman, had thought about a bubble in that respect but the large 'ball' appears to be consistent in relief application and colour on the surface of the coin as say the island to it's right. The small 'dot' mentioned is of the same size and shape as those left and right on the coin which separate the top and bottom wording. As mentioned before, traced a paperweight made in the IOM, nearly identical to mine, but with later IOM coinage, made there by Quirk Collections. Picture of same att. Again, thanks for your interest.