The C in CA stands for contributing security - meaning there are...

The C in CA stands for contributing security - meaning there are future payments due on the security. The A is merely an identifying letter to separate similar securities in the one company that have different characteristics.

In the case of BCS, the A in BCSCA identifies securities on which only the initial instalment of $1 has been paid, and therefore there is still $2 to pay in future instalments. Once the second instalment (currently due 29 April) of $1 has been paid, the securities on which that instalment has been paid will become BCSCB, whilst securities on which the second instalment has not been paid will remain BCSCA until the second instalment is paid.