As there are a few misconceptions in some of the posts, consider the following which will give you most, but probably not all, of the story for this particular Type ...
(a) the certification bases are given in TCDS
A24CE at pp 23 for the 1900/1900C and pp 37 for the 1900D. As you will observe, tracking down the specific regs
here will take more than a few cups of coffee and, as I drink far too much of the stuff already, I'll leave it to you to do some legwork ..
The speed limits appear to be
(i) pp 21: 1900/1900C (C-12J) Vmo = 247 KCAS, Mmo = 0.48
with the changeover at Hp = 12,600 ft
(ii) pp 36: 1900D Vmo = 248 KCAS, Mmo = 0.48
with the changeover at Hp = 13,200 ft
(b) provision is made for the design organisation to use Vmo/Mmo.
Current reg for reference and you are invited to track down any differences for the specific certification.
(c) talking generally, while the operating envelope is a combination of a number of separate requirements resulting in a set of different limits which are overlaid to produce the envelope, in general, one notes
(i) a constant EAS limit at lower levels where dynamic pressures are limiting structurally - Vmo - which needs to account for ASI corrections .. this minor variation may be omitted to keep the limit simpler for operational use.
(ii) as Hp increases, at some point it is usual to see compressibility considerations become limiting and the ASI limit reduces along a constant Mach line - Mmo
If there is no reducing limit at altitude, that is a reasonable indication that either the speed or Hp envelope is not sufficient to see compressibility matters become limiting.