1921 Shilling Possible Specimen Strike

The 1921* specimens were produced in 1926 from old working dies - as far as I know these were the only 1921* specimen strikes produced. They weren't produced in enough quantity to cause the die cracks so it's logical to assume that the die cracks were already on the die. This means that business strikes are certain to possess identical die markers.

A few comments on your coin having viewed it
- it was struck from the same dies as the specimen (this couldn't be too uncommon as the die had to have been used long enough for it to crack)
- Sydney mint specimens have smooth, satin surfaces, your coin's surfaces are too rough from hairlines to assess this
- your coin has a few rim nicks and contact marks indicating circulation, or at the very least mishandling
- your coin is remarkably well struck up considering it was produced from a later die state.

The last point is the key one, it is unusual for a business strike to be so well struck up from a later die state. It is possible that it is a specimen strike but without provenance or original surfaces it is impossible to be sure. That being said, specimens derive their value from their superior quality which your coin lacks so at best your coin will earn itself a small premium for being very well struck up (relative to other 1921* shillings).