Sentence:  I had written him a letter which I had, for want of better / Knowledge, sent to where I met him down the Lachlan, years ago, / He was shearing when I knew him, so I sent the letter to him, / Just <WSD>on spec</WSD>, addressed as follows, “Clancy, of The Overflow”. 
Meaning of the word marked with <WSD> tags:  In hope of success, using one's best guess