In current lore the bolters used by the Space Marines are standardized at .75 caliber shells, though before the Horus Heresy most of all the marines' bolters only stopped at .70, the Phobos pattern above being one of the last. As the Heresy went on there was a greater need for more punchy ammunition to penetrate space marine grade armor since during and before the more stronger Mk. IV armor was being developed and distributed. This also leads into the reason as to why many marines can be seen wearing pauldrons or leg armor with 'pleasure stud' like pop-rivets, these being in place as a stopgap to mass produce armor during the war with the purpose of each stud acting as a kind of binder to keep the armor from breaking and cracking too much if hit.
As for the armor he's set with a mix between the Mk. II and Mk. III, like with the legging armor being the older and the upper half of his body being clad with the later pattern. As far as the lore and storytelling goes most of the marines would adopt an ad-hoc mindset for wearing armor, like the piece of one set being found more useful than another and using only that single piece instead of the entire set. In current lore this mentality has kinda died down some and most armor sets that may look older are either really old possessions of the chapter or it was just designed cosmetically for throwback purposes. Alot like how when the corvus pattern helmet was developed during the Heresy its design was kept and developed upon into current lore and mostly being used by assault marines.
Oh, a mix! Thank God... no wonder I had a bit of trouble.
In latter days I imagine Mechanicum personnel turn a blind eye to using older-pattern pieces on newer power armor because privately they regard the old designs to be more 'pure' and 'holy.' If I recall correctly, a lot of Marines consider the MKVI Corvus helm the high-water mark, due to the increased autosense hardware in it's 'beak'.
Despite their apparent crudity, a lot of the older suits were of better design; a huge amount of Mechanicum knowledge got lost during the Heresy.