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ROCK/POP Billy Joel: ''The Nylon Curtain.'' (Columbia Records. QC 38200.) Billy Joel starts ''Nylon Curtain'' with a Chevy Belair of a song - a simple, powerful rocker, ''Allentown.'' He continues with a selection of songs and ballads he performs a la the Beatles. Let's avoid the easy comparisons, except to say that Mr. Joel has managed in some instances to pull off some remarkable sonic flashbacks. But in the long run the device seems more like a gimmick than an enriching stylistic twist. Nonetheless, the song ''Allentown'' is good and has a refreshing rawness about it, building powerfully to a climax. The rest of the first side of this LP is a desert of overly intrusive songs best represented by the overblown ''Pressure.'' Joel begins to display his versatility and virtuosity that made ''The Stranger'' so welcome a few years ago on the flip side. Here, he earns the price of the album with the clever ''A Room of Our Own, '' the whimsical ''Where is the Orchestra?'' and other tunes.