Read Write CMOS
The 6818 chip has a British Summer Time function, accessed by setting the "daylight saving enable" bit in Register B, which you might have seen in the previous picture.
Some Hitachi chips apparently don't support this function if they have an "X" or "RX" marking on the chip near the batch code, although I haven't found this to be the case. This function in my util will test whether BST can be enabled.
Curiously, it takes effect on the last Sunday in April, not March, so is useless for the UK, but maybe there is some other territory where this timezoning applies.