Death Spectre

That is based on an extrapolation of the original WD129 article saying that many chapters chose to continue with MkVI rather than adopt MkVII, the same article also stated that many modern chapters make use of MkIII for boarding actions so by the same logic MkIII is also still produced. Note the 'for boarding actions' there are two ways this would work: dedicated boarding squads (which would be a codex deviancy I believe) who wear it 24/7 or else the suits are kept in storage and donned only when needed much like Terminator armour. In which case it makes sense that at least some of the suits would be stored aboard the chapters vessels where - if the Crimson Fists do use MkIII - they would have survived the destruction of the fortress monastery and be available for making up the shortfall - either being issued to veterans or even issued to n00bs. In the novel "Salvation's Reach" Gaunt Ghosts are assisted by 3 Astartes (from 3 different chapters, but...) who help spearhead a what is considered a potentially suicidal raid on an enemy space station. Despite the low chance of survival, they make use of boarding plate armor, together with boarding shields. It is specifically stated that these are separate suits, and they each have another set of armor that they wear before and after the operation. The Forgeworld series on the Badab war also introduces the concept of "Void Hardened" armor. Although power armor can operate in a vaccum, void hardened armor has had additional reinforcement and back-up systems fitted to make it more resilent and effective in that sort of environment. Sort of like Artificer armor for the depths of space. So there is some precedent for what you suggest. Another thing I would point out is that, despite the Chapter Monastery being destroyed by a big explosion, there could have been some salvage recovered later. The Rynn's World novel has a scene immediately after the explosion, in which Kantor has come to and experiences regret that they don't have an Apothecary along to reclaim the geneseed of the fallen brothers. This would imply that there were some organic remains left, and by extension some of their armor could have been reclaimed. I think they end up burning the remains to avoid desecration. It might have required months of digging, but I'm sure they would have gone back to see what could be recovered and reused/repaired. As others have suggested, however, there probably would not have been much. Perhaps a solution would be to use pieces of Mark III or IV (e.g. a helmet, greaves) mixed with VII? There is a precedent for this in C:SM which has a picture of the entire Ultramarine's Second Company: you can pick out bits of Mark V and other marks here and there. Thats what I'm doing with my own army.