I can confirm the problem with 1st level of cowboy world.
I bought original Rockford C64 tape and played yesterday. And I really don't know how to beat this cave. There is an invisible item like on picture up posted by Arno. I guess that it should be a snake who would produce missing few diamonds.
But there is another problem - time is running too fast and it will be still hard to finish this cave.
Also I've found another issues:
I'm not able to collect clock item. He is standing like a wall.
I'm not able to finish first cave of 5th world (fire and water) because I'm running out of time.
Anyone can confirm that? It looks like C64 port has bugs and maybe it would be cool to try to fix them?
In the past I made a re-crack of one cracked version of Rockford and added unlimited time option to beat the level with water and fire. I don't remember yet but seems that this crack has "fixed" cave 1 in cowboy world but still there is not possible to collect the clock item.
https://csdb.dk/release/?id=12701