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RCCheck out issue #7 of Old Cardboard by Tim Newcomb. He does a great job of describing the wide pens. Issue #6 also has info by Tim regarding the R313 set and its variations.
As far as scarcity, the 314B set is readily found but according to Tim is more scarce than the earlier wide pens.
The articles are definitely worth reading and I'm sure other posters will be more knowledgeable than myself and will chime in.
RC