According to the Tangorin dictionary, the frame has two little 'drops' on the left. RTK says different.

http://tangorin.com/kanji/crossing
http://s3.directupload.net/images/100710/gr4ojjri.jpg

what's the 'true' form?

Two drops is the official form, but one drop is a valid alternate form.

Before 2004 it wasn't possible to input the two drop form using JIS (despite it being the official form), so RTK predates it.

Heisig seems to like using only simplified forms whether they are official are not too though, as evidenced by the contents of the RTK supplement.

so there are other kanji with more than one form? can you write the numbers, so that I won't be confused?

Probably 50% or more have multiple forms.

http://en.wikipedia.org/wiki/Kyūjitai (old forms)
http://en.wikipedia.org/wiki/Shinjitai (new forms)
http://en.wikipedia.org/wiki/Itaiji (alternate forms, includes new forms of kanji outside the joyo & name lists)
http://en.wikipedia.org/wiki/Ryakuji (abbreviated forms)
http://ja.wikipedia.org/wiki/俗字 ("colloquial forms", there is no english page)
Many characters also look significantly different when written and printed, ex 令
etc

You're not going to see anything other than shinjitai very often during daily life. A lot of people have kyuujitai in their names though. ex: I know a 澁谷, where the first character is the old form of 渋.

To be more precise, if I understand correctly, the "road" radical used to always have two drops. When the Tōyō and Jōyō kanji lists came about, they simplified the radical to have one drop. Any kanji not on the list still takes two drops (but one drop is always permissible and won't make you look like an idiot). This is the way character simplification works in Japanese. The Japanese are a buncha bureaucrats, so when they had a list of characters to simplify, they simplified only those.

You can sometimes see this phenomenon in ways that are much more visible. For instance, 龍 became 竜, but 襲 was left unsimplified even though it contained 龍. (In this case it wouldn't be normal to simplify 襲.)