Hi, Brett.
Don't worry - Arcadia only make two types of T5 reptile lamps, they are both HO (high output) ... the D3+ (12% UVB) in a box that's basically more orange, and has a beardie picture; and the D3 (6% UVB) which has a green box and a chameleon on it.
If you have the one with the beardie picture, then you have the right tube.
Storing the tube has no effect on the UVB output. It only decays while the lamp is alight. You can unplug it for as long as you like, and it will carry on where it left off.
The output of all high-quality UVB tubes will decay quite rapidly over the first ten days, then progressively more slowly until after about six months, the rate of decay is very slow indeed. With high-quality tubes like these, there is then very little further decay for the rest of the year.
So... no worries!
Frances