Bumpers — Throughout its production, Conv. Ds came with either the "Euro", shorter bumper guards without overrider bars, or the North American taller bumper guards combined with the overrider bars. The North American style low (155mm) bumper guards(aka overriders) and bars(aka protection tubes) were used from the beginning of the Convertible D production in August, 1958 until January, 1959 and changed to the high (190mm) bumper guards and bars as of February, 1959, according to the factory's Spare Parts Catalog III and the Supplements to the Service Manual.
Remember that the factory did not release 356s in exact chassis number order, so some later chassis numbered Ds during this changeover period might have the low style even when earlier numbered Ds had the high style. This inexact chassis number release holds true for all changes in all 356 models.
Door Wedges — Elimination of the lock post door wedges (shown at right) took place similar to the other T-2 356A models. Conv. D owners report around mid to late April, 1959, so far. More information here.
Door Top/Roll-up Window Openings — there seems to be a difference in the style of the door tops at the roll-up window slot. This change could have taken place at the same time the door wedges were removed, but more research is needed. More information and photos here.
Front Turn Signals — The front turn signal differences are very well documented in the 356 Registry article Front Turn Signal Types. It seems that two basic styles of front turn signals might have been used during the Conv. D's production: the earlier wedge with the screw-on chrome bezel (stamped SWF K2665 on the bezel) and the slightly later version with only a wedge and the new twist-on (Bayonettverschluss) lens (stamped either SWF2665 or SWF K12627 on the chrome wedge). The Spare Parts Catalog III mentions that this later twist-on style began at Conv. D #85503, however this does not mean that the earlier style was not used on the Ds as evidenced by observations of original early Ds. More information is always welcome.
Rear Interior Upholstered Panels — the use of the rear upholstered panels (shown at right) was discontinued sometime around Dec., 1958 through Jan., 1959 as reported so far by D owners. More information here.
Top Frames — There was a change to the top frame design at some point during Conv. D production (shown below), with an extra support piece added on each side to prevent it from folding at the rain gutter split.
Transmission Types — The 644 transmission was slowly phased out and replaced by the 716 trans throughout the Conv. D model. So far, the lowest chassis number known to have been built with the 716 gearbox was #85792, but the 644 was used until nearly the end of D production.
Windshield Frames — Conv. Ds were only given the brass windshield posts; the first pot metal posts were used on 356B Roadster #87316, according to the 356B Spare Parts Catalog.
Windshield Heights / Shapes — According to the Spare Parts Catalogs, Conv. Ds used windshield glass part no. 644.541.101.41 and Roadsters used part no. 644.541.101.45. Apparently there was a 7/8" taller windshield also produced later in the Roadster production, so more info is needed. There has been a lot of discussion about differences in glass size, see 356 Registry's 356Talk archives. One person mentioned that at the factory a glass installer had the benefit of trying several glasses to get the best fit on both Speedsters and Roadsters.
Windshield Frame Tension Rods — Conv. D chassis numbers 85501 – 86530 used a hook style windshield frame tension rod (#644.541.025.41). Chassis number 86531 began the use of a two-screw style tension rod (#644.541.025.45) which was used through the Roadster production, according to Spare Parts Catalog III. The first photo below shows the earlier style, the second photo shows the later style.
Wiper Arms — According to some Conv. D owners, at least two styles of wiper arms may have been used on the Ds. Early Ds might have used the "pickle fork" external two-hook style and later Ds seem to have used the "Conv. D/Roadster" style wiper arm. More research is needed.
Please email me directly at email@example.com if you can contribute to any of the above categories or have found additional differences during the Convertible D production. Thank you for your help, John