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A pictorial approach using Corel Draw: the yellow circle is a 100mm reference with no allowance for foreshortening, red circle is an average of outer and inner perimeters of the bolt holes (line thickness was later increased to make it visible in the photo)
Most of the error is due to the camera having a relatively short focal length, but measuring your photo and scaling gives a BCD of 143.7mm - say 144mm with an error in measurement of 0.2%, which is probably as accurate as you'll get with a tape measure.
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MrE
Cheers
I think it's a 144BCD