Hi,
I'm not discussing personal preferences. To each his own of course as to what he wants. To each his own even regarding what he personally is willing to pay or wanting to pay; that preference will interact with the market either with success or without. So be it. Not my point though. I'm speaking about markets, not individual desires or preferences.
Note that I did not say some caps do not receive more attention or value. I noted that in the entry level for a majority of caps and pens, double jewel vs single jewel overwhelmingly blows away most cap-pattern differences. This is particularly so within a given metal. A basic double jewel pen in basic color is worth probably 2.5- 3 fold the value of a basic color single jewel pen (150-250% bonus for true double jewel) on the retail market. Perhaps 4-5 fold on the wholesale market. A basic single jewel pen is unlikely to see 10-20% price variation (not 150-259%) depending on cap pattern... for most caps within individual steel, gold-filled, sterling cap-metal categories. That difference is exaggerated at the wholesale level, as dealers generally must stretch more (still with more per pen cash profit but lower percentage profit) to buy double jewel pens. This is not preference. This is market.
Parker 51 lends itself to greater (but not absolute) flexibility regarding originality than do some series. Let's take just 1941-48 Vac era, since double jewel is not an issue later. There are a huge number of caps. Most likely were not made all 8 years, but data tend to be lacking-- with some exceptions-- as to which cap patterns were made in exactly which years. There are hints that can give us a sense of early or late, but those are just hints.
Some patterns more often are found with solid gold trim. Recollection is that gold trim was an option. Most often it is found on double jewel pens. Moving a gold clip from a SJ pen (where it might not be original in any case) to a more appropriate (in recognized collector context) pen is no less original than what might have turned up in the wild.
In any case, there is a good chance that when an old 51 trades hands in the modern world, that it already has been subject to swaps.
Upshot. Personal value might be in the eye of the beholder. Acknowledged market value is not.
Of course I could be making this up, just a wee dabbler n' all. Or, I could be the fellow who has made what might be the largest individual dealer's retail market online for these pen in the history of the hobby, with hundreds of pens offered at once, with years of watching said pens sell, having bought hundreds more at pen shows in order to place on the retail internet market. Or sumthin'....
regards
david