Hi Steve, I have been wondering about adding blast bags on my Tirpitz build and I still haven't decided what I am going to do. As you say there are lots of reference photo's with and without the blast bags fitted. The best references with the most photo's I have been able to get hold of are Polish books:
AJ-Press Monografie Morskie [18] Bismark Tirpitz cz. 4, isbn: 83-7237-077-X
AJ-Press Monografie Morskie [19] Bismark Tirpitz cz.5, isbn: 83-7237-078-8
Unfortunately most of the text is Polish with an English translation only added for the reference photos so I have no explanation for the appearence and disappearence of the blast bags.
How do you plan to make blast bags if you do decide to fit them? I have bought the grey/yellow Milliput to brew up something. Most photos of the blast bags show them to be very short though. The bags on the secondary armament seem bigger.
Cheers,
Julian