spat
.700 Nitro Express
Also, as with detent pins, I would be careful of springs that like to put themselves into orbit....I did a quick google search and came up with the same info on a couple different sites. It makes sense too. If the manufacturer included the three jets, you would think they would also include anything else needed for the conversion. If they didn't include an LP spring, it would seem there isn't a need for one.
Is there a black cap marked NG on your regulator? That would be a good sign. You could prove it 100% by unscrewing the cap and seeing if it says LP on the other side. You may want to wait until warmer weather to try this in case something goes wrong. I would also turn off the gas when removing that cap. Not because there is going to be any gas under the cap, but because you'll be releasing the spring pressure on the diaphragm that normally works against the gas pressure on the other side. No need to put any extra stress on that diaphragm.