Klaus, when shopping some places will call it "old style" or "short style" in the product description for the shorter ones. Obviously the one you don't want.
Most of the time though nothing is mentioned in the product description and that usually means its the longer one you will need. They might describe it as "long style" but usually not.
YMMV so when in doubt email/call the place you are considering ordering from. Also make sure they have a liberal return policy. Most do. The only downside to ordering online is waiting for the UPS guy to show up. If you in fact did order the wrong item then you have to wait to ship it back, wait some more for a replacement etc...... Kind of a pain in the you know what.
In the end though on average ordering online is worth the risk. Its typically cheaper than dealing with a local place but in some instances going local saves you considerable time so its up to you. With the frenzy right now many online resources are backlogged and you might have to wait a while for your stuff to ship. At least local if they have it on hand you walk away with something right there on the spot.
Unless you don't have a decent number of local shops in your immediate area to check out then utilizing someone online is your best bet. Also don't hesitate to look in the buy/sale/trade area of the forum here and elsewhere. Lots of folks are in your boat and either order the wrong item themselves, or order something and then realize its just not for them. If they wound up getting it from a place that isn't worth hassling for a refund/exchange they sell it to other folks at a considerable discount. Over the years I've acquired many things this way, not just gun parts either. PC, guitar, RC hobby, etc... and its a great way to experiment on the cheap.
Just throwing it out there for you to think about. Also if you wind up with something incompatible its an option to trade for what you really needed or help the next fella save a few green backs with a screaming good deal.
Best of luck.