Terminated: unknown
It can run either on batteries or using a power adapter.
The switch with the "◠"-symbol, when shifted to the right, causes all calculations to be rounded to two decimals (for currency calculations).
The slider on the right, when shifted to the "m" position causes all calculations to be added up to the calculator’s memory. Use the "rm"-key to see the result. The "cm"-key clears the memory.
When looking for more information on this calculator I came across this site dedicated to ANITA calculators: http://anita-calculators.info (link validated 2024-08-25). A page dedicated to the ANITA 811 model is here (link validated 2024-08-25). This site is by Nigel Tout, who also created http://vintagecalculators.com (link validated 2024-08-25).
Apparently, Sumlock Comptometer Ltd. is an associated company of Bell Punch Co. Ltd, which was a British company. The tag on this ANITA 811 lists its location as being in Uxbridge, Middlesex.
Also interesting to know, this calculator design was shared with TRIUMPH and ADLER and its sliding design is present in several of their models, some of which are in this museum: The accuron 831M also uses the same sliding design but I can not find a link between accuron and either Sumlock or TRIUMPH/ADLER.