If your coin has a different reverse from those below then you may
find a match in the Mintmarks section.
the Ch'ien Lung section of Y.K. Leung's web site.
25 mm and 22.5 mm (slightly different style)
The coin pictured here is very worn, making the mintmark very hard to see.
This coin is somewhat thicker than the other Ch'ien Lung coins shown here. The metal, instead of the common bright brass, is a very red colour. Also unlike the other coins here, the Manchu mintmark for the Yarkand Mint does away with the Boo character on the reverse left.
This coin is a "Shan Lung" commemorative issue. Note the different style bottom (Lung) character.
Coin identification page