Investors hunting for clues to the iPhone X\u2019s reception can take a deeper look at its main manufacturing partners. And the latest doesn\u2019t look good.\r\nApple Inc.\u2019s five largest device assemblers reported a sharp slowdown after peaking at the end of last year, suggesting demand for the high-end device may have faded just a quarter after its release.\r\nWhile Hai Precision Industry Co., Pegatron Corp. and three other key suppliers reported an 8 percent rise in their total sales across the March quarter, growth slowed sharply later in the period \u2014 a drop that in the past has presaged a downturn for Apple.\r\nThe concern is that the iPhone X, while enjoying a customary holiday quarter spike for new-generation Apple gadgets, fizzled out rapidly. Apple\u2019s costliest smartphone has struggled to draw customers in emerging markets, while competitors from Huawei to Xiaomi roll out more premium phones and dominate China \u2014 the U.S. company\u2019s biggest foreign market. On Friday, Morgan Stanley cut its estimate on iPhone shipments by 6 million, underscoring the growing unease since Taiwan Semiconductor Manufacturing Co., the maker of iPhone processors, issued a disappointing outlook that triggered a 7 percent loss in Apple\u2019s value over just two days.\r\nApple\u2019s sales growth bears close correlation with that of its main quintet of assemblers, which depend on the iPhone maker for their own business growth \u2014 Hon Hai alone gets about half its revenue from Cupertino. While it\u2019s difficult to translate their numbers directly into Apple\u2019s, a look at reported figures since 2016 suggests it\u2019s possible to draw certain conclusions at the general health of the U.S. smartphone titan\u2019s top-line.\r\nThe group is responsible for finishing most of Apple\u2019s gizmos: Hon Hai, Pegatron and Wistron Corp. assemble iPhones. Hon Hai also has a role in other Apple products, splitting MacBooks with Quanta Computer Inc. and sharing iPads with Compal Electronics Inc. Quanta and Compal also make Apple Watches.\r\nHon Hai and crew can indeed offer insights into Apple\u2019s sales but to a limited extent, said Jusy Hong, a director with IHS Markit. While Hon Hai, Quanta and Pegatron both rely on Cupertino for more than half their business, Wistron and Compal are far less dependent on Apple, according to data compiled by Bloomberg. In addition, as hardware mavens, their operations would have no impact on sales of software and services, a significant and growing part of Apple\u2019s top and bottom line.\r\n\u201cIt\u2019s true that they are major assemblers of Apple but at the same time they have other customers,\u201d he said in an email.\r\nInvestors remain concerned that iPhone sales at Apple, which reports results May 1, failed to meet their lofty expectations. Mia Huang, an analyst at Taipei-based research firm Trendforce, estimates that overall iPhone production volumes grew slightly to 54-56 million units in the March quarter \u2014 barely up from 52 million in the same period of last year, when it was propelled by demand for lower-priced and older models like the iPhone 6S and ramp up of the iPhone 7.\r\n\u201cAccording to our estimates, iPhone X\u2019s production volume fell by 50% in the first quarter compared to the fourth quarter,\u201d said Huang.\r\nStill, the iPhone X\u2019s higher price tag ensured a tidy jump in revenue. For now, analysts are still counting on the first three months this year to have been Apple\u2019s best second quarter ever with an average estimate for $61 billion in revenue. And while its assembly quintet saw sales decelerate during the period, their collective growth is still double that of a year earlier.