The numerical sequence suggests the number part is 63, because the other numbers are successive multiples of 7. The letter d is repeated for 21da and 28do, where 2 is also repeated, so d may represent 2; the letter a is repeated for 42aa and 49am where 4 is also repeated, so a may represent 4. There is inconsistency, but since 6 only appears in 56ml, perhaps l represents 6; and since 3 only appears in 35ta, perhaps t represents 3; so lt represents 63 and the next in the series is 63lt.