This looks interesting.

[Redshift]

I have never seen one like that before. Brilliant idea as long as the cylinder is not too big. Is this a single use cylinder made to fill the SMB with the exact amount of air needed?

No, this is a reusable aluminium cylinder and there are different sizes. I don't own an SMB of this type, but I think the small cylinder has a DIN or a-clamp attachment so you can refill it directly from any cylinder.

---------- Post added March 8th, 2015 at 05:12 PM ----------

Several here Buy Mini-Cylinders, Valves & Spares | AP Diving for example.

 

[Rich Keller]

I assumed the cylinder was reusable. What I wanted to know is the size of the cylinder just big enough to fill the bag completely one time?

 

[Redshift]

Oh, sorry, misread what you had said.
There are several sizes and they can be filled up to 232 bar. There are also different sizes of SMBs that can use them. And it will all depend on the pressure (depth) you're at.
To send and SMB up, only a little bit is needed as it will expand. People open a bit, close it and let it go. If you are thinking of using it as a lift bag that may need more air at depth, it depends.
AP sell 0.4 l @ 232 bar and 0.35 l @ 300 bar mini cylinders that fit on these SMBs. But they also sell just the valves, so you could use them on larger cylinders if needed, like those 1 and 1.5l used for argon. But it would become something not so easy to carry.

 

[Rich Keller]

If you are only using it as a SMB then a cylinder just big enough to fill it completely would be all that is needed and there by reducing the overall size. With the added benefit of the weight of that cylinder helping to hold the SMB upright at the surface. Also in an OOA emergency you can still inflate the thing and ride it to the surface if need be.

 

[Akimbo]

The trouble with a 75 Lb bag is it guarantees that all you will find are 100 Lb anchors. I think that is a corollary to the rule that states that divers only see white sperm whales swim by when their macro lens is installed.

Just carry a smaller easier to use SMB and come back for it with the boat.