Walther is mostly known for its later electronic calculators. Their most common mechanical model is the WSR-160, or Muldivo Mentor, with its very distinctive rounded body shape. The price for these is usually around £40 (see below).
For model identification, see http://www.rechenmaschinen-illustrated.com/pictures_Walther.htm.
For dating based on serial numbers, see https://www.ifhb.de/Aktuell/Walther-Reese.pdf.
http://www.johnwolff.id.au/calculators/Walther/Walther.htm
https://www.rechnerlexikon.de/en/artikel/Walther
https://www.xnumber.com/xnumber/walther.htm