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By Tim Hepher and Valerie Insinna

(Reuters) -Boeing Co said on Sunday it will have to do more work on about 50 undelivered 737 MAX airplanes, potentially delaying near-term deliveries, after its supplier Spirit AeroSystems (NYSE:SPR) discovered two mis-drilled holes on some fuselages.

Boeing (NYSE:BA) confirmed the findings in response to a Reuters query after industry sources said a spacing problem had been discovered in holes drilled on a window frame, but the jetmaker said safety was unaffected and existing 737s could keep flying.

"This past Thursday, a supplier notified us of a non-conformance in some 737 fuselages. I want to thank an employee at the supplier who flagged to his manager that two holes may not have been drilled exactly to our requirements," Boeing Commercial Airplanes CEO Stan Deal said in a letter to staff.

"While this potential condition is not an immediate flight safety issue and all 737s can continue operating safely, we currently believe we will have to perform rework on about 50 undelivered airplanes," Deal said.

The checks focus on potentially incorrect positioning of two holes on a window frame assembly supplied by Spirit, a condition known as "short edge margin," the sources said.

As of Friday, the "non-conformance" or quality defect had been found in 22 fuselages - nearly half of the 47 inspected up that point in production systems spread between Boeing and Spirit - and may exist in some 737s in service, they added.

The figures Deal supplied to employees on Sunday suggest that the inspections proceeded rapidly and that the problem affects a minority of the hundreds of fuselages in the pipeline.

"As part of our 360-degree quality management program, a member of our team identified an issue that does not conform to engineering standards," a Spirit AeroSystems spokesperson said.

The findings came to light in a routine notification known as a Notice of Escapement, in which suppliers notify Boeing of any known or suspected quality slip, the sources said.

Such quality reports are common in aerospace but the discovery comes as Boeing and its best-selling jet face intense scrutiny after the mid-air blowout of a door plug on an Alaska Airlines jet on Jan 5.

Exclusive-New quality glitch to delay some Boeing 737 MAX deliveries   1