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Paddle headstock with closed gear tuners is a lot heavier than a slot head,

There's a little less wood in a slot head - perhaps 6% less on mine - than a paddle head, but as John mentioned, 3-on-a-plate tuners will run a little heavier, so pretty much a wash. That said, the real reason why we still see slot heads on twelve frets is for purely aerodynamic reasons.

When I studied under aerodynamicist John Anderson at the University of Maryland's college of engineering, he routinely insisted that seniors taking his high speed aerodynamics seminars support his efforts to catalog the vast number of documents residing in the college's archives. As both a graduating senior and as a Tau Beta Pi member, this sort of work was part and parcel of escaping undergrad studies with a fellowship-worthy GPA and recommendations.

While working through a water-damaged box of letter characterized as 'Misc. Corr. - Industrial', I happened upon original copies of correspondence between F.A. Martin - then head of the Martin instrument operation - and several engineers working for the Martin Aircraft Company. This correspondence was part of the tens of thousands of items which as of 1989 was still uncatalogued in the Glen L. Martin collection owned by the University of Maryland (the university's college is named for the aviation pioneer and benefactor), and other than the coincidence of identical company names, must have been unremarkable at the time. As a guitarist who had owned several Martins and restored my first vintage Martin at age 16 (I reglued the bridge on a garage sale 00-17), I read through the thin, water stained, and largely illegible correspondence file with interest once I noticed the sender's address and company name.

From what could be read of the documents (not a little bit moldy and water stained), when Martin went to the longer neck and shorter body of the Orchestra Models (as all 14 fret Martins were referred to even into the 1950's), the initial feedback from Perry Bechtel was that the then-slotheaded OM would feel unbalanced in the breeze created by the horn section behind him (dance bands were quite common at the time, and guitarists were often relegated to the same unfavorable position fronting the horns as had been the practice with the often reviled banjoists before them). F.H. Martin - then head of the company - happened to mention this to his shop foreman, leading to a visit to the bench of a recent hire formerly of the NACA model shop. The employee - his name is unfortunately lost to history - mentioned that he had built many models for use in wind tunnels to explore the effect of placing narrow slots in the leading edges of wings to alter their low speed lift characteristics. The worker recalled the names of the engineers at a Baltimore-based aircraft company that had participated in the research, and F.A. proceeded to correspond with these gentlemen for several months in 1929. The worthy engineers at Martin Aircraft concluded that elimination of the tuner slots in the peg head - coupled with the longer neck moment of the 14 fret-to-body instrument - might just reduce the issues with aerodynamic balance noted by Mr. Bechtel.

F.A. had the gentleman run up a neck with a solid peg head to see what the differences might be, and that second OM prototype quietly replaced Bechtel's slot-headed original. Further efforts suggested moving the tuner keys to an orientation in the horizontal axis, creating the modern Martin configuration which other manufacturers were already using for purely economic reasons. The elimination of the slots and change to vertical shaft tuners produced just enough of a change in aerodynamic force to balance the reshaped body in professional orchestra use, and the last note in the file was from F.A. to one of the engineers lamenting the current economic troubles (how times change) and offering a factory tour should they find themselves near Bethlehem and at loose ends for something to do with their time.

So there you have the reasons for the use of solid peg heads on the 14 fret-to-neck instruments. Given Martin's famous thrift in both ornamentation and in features deemed superfluous (cutaways, fancy inlays between the end of the World War and the mid-sixties) it always struck me as odd that Martin had kept what was essentially two separate neck designs, despite the fact that at the time, the slot head would have seemed old and dated to buyers (the wave of nostalgia for all things vintage was nowhere in evidence in the late 1920's). As it turned out, there was a perfectly logical reason for the practice, just as we see for the vestigial dart on the back of the D-28 and D-40 series guitars. I would have come forward with the information years ago; however, I had totally forgotten the research until recently - while cleaning out my dad's attic and going through the notebooks from my college years, I found both the research note and Dr. Anderson's reply to me - "Stick to aerodynamics and leave the guitar stuff to the kids at the Food Coop. Thanks. Prof. A."