What do I look for in used records?

Wdi...You can tell if a record is stereo or mono by the appearance of the grooves. Grab two records that you know are stereo and mono, and see what I mean.

Good general info from the folks above. LM-1903 is mono, as Pmotz indicated. LSC-1903 was the stereo version. "New Orthophonic" was RCA-speak for the superior equipment and techniques they developed just before the stereo era began in 1954. They used the phrase on both mono and stereo releases. As others have said, RCA records from this era were among the best ever made.

Do not assume that your mono pressing is necessarily inferior to stereo. Quite the contrary! Most 1955-65 RCA recording sessions were taped simultaneously on two seperate machines, one for each format. Since the mono machine used the entire tape for one track, it often captured inner details and dynamics better than the stereo setup. I have an RCA mono LP from that era, Arthur Rubinstein/Beethoven's 5th concerto. It lacks the width of stereo but it gloriously reveals the shadings of AR's playing and a million orchestral colors.

Entry level rigs and systems will not reveal everything that's on such a fine record, but that doesn't mean playing it won't be thoroughly enjoyable. Just take care your stylus is undamaged and your setup is accurate. If that record's clean it's a real treasure, and of course you already know how good the performance is. I have a current reissue on audiophile vinyl. Mine cost $27 but if your's is undamaged it may actually sound better. Good find!

Having bought several hundred used records in the past year, here are my observations.

I have looked closely at my used records which have persistent noise under a microscope and found that they have a slight roughness or tearing to the otherwise smooth groove walls that was presumably caused by damage from a bad needle. These are LP's that to the naked eye appear in excellent condition. The noise you will hear is a slight static or distortion at the more intense points of the music. Unfortunately, this type of damage is very hard to observe unaided but it seems to be quite prevalent. Visible scratches may or may not be a problem as many are just along the record surface and do not impact the needle as it plays deeper in the groove.
I agree with the posters above that stereo recordings are not better than their earlier mono versions. In general, I have found that the reissues from the early 80's on, even the ones claiming to be "audiophile" versions are inferior to earlier recordings. To me they just sound like the CD with the analog noise. They lack completely the "presence" that makes well recorded vinyl superior to digital. This unfortunately, confirms that vinyl is dead except for those who, like me, want to sour the earth for real old records. If you're planning on making up your collection from vinly recorded after 1984, I suggest you just go with high quality digital.

Thanks everyone for the detailed help. How is the quality with some of the other labels I'm seeing; such as, Angel, Tops, Vanguard, Vox, Westminster, and Seraphim?

By the way, I went back and picked up 7 more:

Brahms Sonata no. 2 - Piatigorsky/Berkowitz - Columbia ML 2096 (This one is a 10" disc, Long Play Microgroove, whatever that means)

Ravel Concerto in G - Munch/Schweitzer - RCA Victor LM-2271 - 1959

Sibelius Symph. 6&7 - Karajan - Angel 35316

Tchaikovsky/Moussorgsky - Mitropoulos - Columbia ML 5335

Respighi Fountains/Brazilian - Angel 35405

Beethoven Pastorale - Stern - Tops L-1618

Rimsky-Korsakov Scheherazade - Rossi - Vanguard SRV-103

I'm not seeing dates on most of these.

Unfortunately, these will probably just sit on a shelf until I can spring for a phono stage and perhaps a new cartridge/stylus.

Obviously it varies from record to record, but here are my impressions based the few in my collection. Serious collectors could tell you more.

Angel - okay to good
Tops - thought they made baseball cards
Vanguard - okay
Vox - okay to good
Westminster - easily the class of this list; glorious mids, detail and presence; older records have restricted frequency extremes but are still wonderfully alive
Seraphim - same as Angel, more or less