This inside the collar felt, can be Nuno Felting. Often used in the fashion industry. Along with those layers of vegetable tanned lamb's skin, double layered slowly yielding to shape. With buckle systems requiring steel lined holes in those straps, because of the extreme softness the leather will exhibit.
This is the link to the article to Nuno Felting, complete with referencing to other sources.
And this felt can be made to a Crimson Red, to draw the eyes to the collar. With the inner two layers being made wide for greater comfort, such as with a three inch wide or wider collar. Along with from one inch up to two inches up from the lower edge, those outer two layers can exist. With the breaking in process allowing a deforming of the wide inner double layer for comfort. All while a felt separator helps distribute the force from, those outer two layers.
The liner felt can feel soft like silk, even though wet felting can be used to create the spacer felt. With spacer felt for preventing stresses from excessively thickly stacked lamb's wool, from having its fiber used to bind it such as paraffin coated flax fiber from ripping through it. And these buckles will have a metal cylinder to roll across, instead of rub across the lamb's skin straps.
Those lined collars can be silky smooth on the inside, because of applied use of felting. And this will hinder the blood circulation reduction caused by, finger grabs and choke holds to the throat. Along with reducing blood vessel constriction in the neck, because of circulation near the spinal column. Yet vegetable tanning is for a special softness just like lamb's skin, with both together being a custom order.