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yes, on the bar worn by your cycle the centre is an external sleeve
in this image the edge of the sleeve can be seen clearly -
View attachment 1488781
Ets Rene Chayale also produced bars where the centre has a barely perceptible bulge. here is one marked Kiprim -
View attachment 1488782
cannot answer with regard to dimensions as have no hands-on experience working with Chayale stem/bar products
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question for you -
have no experience working with a model 630
you mention tubeset as being "full 531"
on French production cycles with eleven tube nominal 531 frames it is common for manufacturers to "cheat" and employ a NERVOR steerer
Peugeot, for example, used 531 while Mercier and Lejeune at this era employed NERVOR
if you should have occasion to repack the P3 will be curious to hear what steerer is worn by the 630
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