German sandal maker Birkenstock is once again expecting a significant increase in sales for the current financial year, forecasting sales through September to rise by 17% to 18%. The forecast was included on Thursday in Birkenstock’s first quarterly financial report since the company went public on the New York Stock Exchange in October. Birkenstock shares in October began trading at below the issue price of $46 and took seven weeks to reach that value again. The company’s shares have generally traded above that mark since then. The figures presented on Thursday are for the company’s financial…