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Ahead of Apple's quarterly earnings report on Tuesday, analysts agree on one thing: The latest iPhone iteration will not deliver the mega sales enjoyed by the iPhone 6 line around its launch.

"The evidence thus far suggesting that demand for iPhone 6s is not as frothy as last year is tough to ignore," Susquehanna Financial Group analyst Chris Caso wrote in a note on Thursday.

Caso, who has a positive rating on the stock and a $155 price target, has reduced his estimate for iPhone fourth quarter sales from 79 million to 74 million, representing a year-over-year decline of 0.6 percent. That estimate is just slightly below the consensus of 75 million. Apple derives more than 60 percent of its revenue from iPhone sales, so investors watch the number of units sold closely.

The company said it sold 13 million iPhone 6s and 6s Plus devices during its first weekend of sales — 3 million more than in the same period last year — but analysts point out that this year's number includes China.