The financial instrument in this case is the class of puttable ordinary shares issued by S Limited to T Limited. The shares have the following features:

1. T Limited has the right to put the shares back to S Limited for a fixed price, which is consistent with other classes of ordinary shares.
2. T Limited is also entitled to convert the shares into equity instruments of S Limited.
3. The number of equity instruments received by T Limited upon conversion will vary based on the market value of S Limited.

Based on these features, the class of puttable ordinary shares can be classified as equity instruments, as they represent a residual interest in the assets of S Limited after deducting liabilities. The fact that T Limited has the right to put the shares back to S Limited for a fixed price is a common feature of equity instruments, and does not change their classification.

The right of T Limited to convert the shares into equity instruments whose number will vary based on the market value of S Limited presents a potential issue with the classification of the instrument. The fact that the number of equity instruments received upon conversion will vary means that the instrument has a feature of a financial liability. This is because the number of equity instruments received by T Limited is not fixed and therefore subject to variability.

However, it is important to note that the potential liability arising from the conversion feature is limited to the number of equity instruments that T Limited receives upon conversion. As long as this number does not exceed the number of puttable shares held by T Limited, the instrument can still be considered as having a feature of equity.

Therefore, based on the information provided, the class of puttable ordinary shares issued by S Limited to T Limited can be classified as equity instruments with a feature of a financial liability due to the conversion feature.