The necessity of tin straps make zero sense to me, unless it was to help keep the round together in shipping and during travel in the limber. That is the only good explanation I can come up with.
I understand the need for the lead sabot to fill the rifling to keep gas pressure behind the round 'pushing' on the cannister pieces as high as possible for maximum effectiveness and destructiveness through travel.
I agree that imparting spin of the contents keeps them together more as they travel through the air, which as mentioned earlier, isn't necessarily a good thing [ depends on the range of your target though ]
As a plumber of the old school, I know how difficult it is to join the two different materials of cast iron and lead.......Back in the day, we would take cast iron, heat it up with a torch, apply flux to clean it and prep the surface, then use a bar of lead that is actually 50 % of tin antimony and 50 % lead and melt that onto the prepared, hot cast iron, using a special stiff cotton fabric with melted candle wax on it as a trowel or spatula to distribute it evenly. We actually had to melt the wax onto the fabric ourselves each time we used it.The lead wouldn't stick to the fabric with the wax on it. We called this 'tinning' it Then we would carefully, very carefully, use a torch to gently melt the lead into the tinning on the cast iron, adding more solder as necessary to create the bond. The lead we worked with was only maybe 1\8 inch thick, so a thick dense sabot would be much more forgiving. We had to be able to do this in the field, and I had to demonstrate it to get my master plumbers license back in the last century sometime. Thank goodness for pvc !!
Anyway, I can see why attaching a lead cup style sabot to a rounded bottom cast iron shell would require the tin straps to help hold it all together - loose the sabot and your projectile's flight is severly effected. But, I have no idea why they would use tin straps on a cannister \ shotgun style round, unless it were to hold it together for shipping.
Sorry for the long response.....