It seems for us OCD minded folks there is a lot of discussion concerning crystal specs especially when it comes to the 6105 divers. I have pondered this question myself many times and finally, when time allowed, sat down and measured 20 nos Seiko 320W10GN crystals with a high quality digital micrometer. Here are my findings:
Crystal Height:
Type 1: Single dome, flat top measures 3.81mm to 3.89mm
Type 2: Double dome measures 3.76mm to 3.85mm
Type 3: Double dome measures 3.57mm to 3.70mm
Type 4: Double dome measures 3.48mm to 3.52mm
An original type 3 measures roughly 3.65mm in height give or take a few mm's. This is from the top center of the dome to the outer base of the crystal where it meets the gasket. Jonathan's type 3 also measures roughly 3.65mm in height.
Here's where a difference comes into play between Jonathan's
(SK's) type 3 and the original 320W10GN type 3:
The height of the crystal at the edge where it meets the bezel on an original type 3 is roughly 3.55mm causing it to sit approximately 1mm above the edge of the bezel. On Jonathan's it's 2.85mm. So, although the overall height on Jonathan's (or SK's) is correct, the height at the edge where it meets the bezel is incorrect causing it to sit lower then the original in reference to the edge of the bezel.
Seiko's Type 2 edge height is roughly 3.60mm causing it to sit approximately 1.5mm above the edge of the bezel. As you can see from the measurements above the type 2 crystal is very close to the type 1 in height. The main difference is the type 2 is a double domed crystal rather then a flat top crystal.
Regarding numbers and popularity, out of twenty nos Seiko crystals I found the following:
3 Type 1: Single dome, flat top measures 3.81mm to 3.89mm
4 Type 2: Double dome measures 3.76mm to 3.85mm
10 Type 3: Double dome measures 3.57mm to 3.70mm
3 Type 4: Double dome measures 3.48mm to 3.52mm
Based on these findings they collaborate what Jonathan believed and shared on March 9th 2014 shortly before his Passing. That the most popular 320W10GN crystal is the type 3.
Here is what Jonathan shared:
" I did a lot of research on the 320W10GN00, which has four variants over its run of nearly fifteen years. The second and fourth variants are insignificant as they were not made any any appreciable numbers.
What I now call "Type I": The oldest (correct for pre-1972 models) has a flat top and inside dome. It measures 3.85mm high with frosted sidewall and polished bevel. The crisp edge of this crystal is a chip magnet. In the early '70s I remember seeing fairly new watches coming in for new crystals, within weeks in some instances, due to chips and nicks on the top edge. My assumption is Seiko got tired of these complaints and dropped the height by 0.20mm. This doesn't sound like much but it's enough to get that edge closer to the bezel of a sport watch or the rotating ring of a dive watch. Also, flat-top crystals are terribly reflective. Double domes are far less so which is why this makes sense for a dive watch. Also, the double dome is much more structurally sound than flat/flat or flat/domed.
What I refer to as "Type III": Third is double domed but is only 3.63mm +/- 0.02mm high with frosted edge and polished bevel. This is the most common variant and after 1972 became correct for both OEM and service replacement applications. Thus, if your 1970 watch has the "Type III" crystal, it was replaced as part of a service after 1972 and IS STILL CONSIDERED CORRECT."
www.thewatchsite.com/21-japanese-watch-discussion-forum/88250-6105-crystal.html
So, although there are differing opinions about which original crystal was the most popular I'd tend to side with Jonathan's original thoughts: The type 3 was the most widely used original crystal.
Unless it's a very early 6105, it's nos Seiko type 3 for me........
In the absence of an original, I'd have to say Jonathan's (SK's) reproduction comes closest to the original and would be my next choice. as inquiring minds would like to know, I'd be curious to know how SK's type 2 measures up...........