Co/Co, Mh/Mh and Di/Di will do that on a wheaten base, for example:
eWh/eWh + Co/Co mh+/mh+ = Co alone will turn the gold tone to a buff color and will leave neck pattern, Wheaten being the least of the melanized e allele will not allow for saddle pattern on columbian base for example this delaware rooster(eWh/eWh Co/Co, I know its a silver based but the black and white contrast help us see the lack of pattern on the saddle, the lack of pattern on saddle is due to wheaten as opposed to columbian based on eb like wyandottes)
View attachment 2030499
New Hampshires males lack the neck pattern they are also wheaten and Columbian restrictor so I suspect Mahogany(inherited from RIR ancestor) and Di not only cancel each other to make it buff instead of red but since both are columbian restrictors they remove/restric the neck pattern
NH rooster
View attachment 2030500
But I must again go back to the point of not using the males as basis for detecting if its Co or Db as both have about the same effect on males, its the females/hens/pullets that we must focus on this subject, here are Bevs Maran hens, you can clearly see the neck pattern, Db does not produce such pattern
View attachment 2030501
Db females should not have any pattern on the hackle but on the main body as shown by the Ginger OEG females(perhaps less pronounced or all together gone due to wheaten)
Any pictures of the American lines of BTB Marans Females?