Old type Smiths speedometer drive

[Invicta]

Hi , can anyone please help with some information as to how to dismantle the old pre torpedo speedometer drive. I have taken several of the torpedo type drives to pieces, but the older flat ended type is different. I have searched the internet and can find nothing on how to take them apart. I can remove the wheel gear, as that is the same as the more modern ones, but how does the worm gear come out. Is the flat brass end piece screwed in? The torpedo type are simply pressed in. The reason I want to take it apart is to check the wheel and worm to see if they are correct for a 2 to 1 ratio. I bought a box of assorted drives on e bay to see if I could find one to fit my 1949 Model 18 . I selected one with the smallest hole in, and had a stepped spacer turned to fit. It has 2-1 cast on the housing, but the felt seal holder is stamped 19-10 . clearly it has been apart at some time and maybe no longer contain the original sized internals to give the correct 2-1 ratio. Also the drive that I selected is designed to fit with the cable coming out vertically. Is that correct, or should it be a drive where the cable comes out horizontally and runs along under the frame?

[Invicta]

If anyone wants to know The brass end cap on the old type of drives unscrews, it is a normal directioal thread. The worm gear can be pulled out from the front with needle nose plyers. After removing the 4 rivets the main gear can be removed. If taking one of these speedometer drives apart be careful not to loose the small washers that fit on the worm gear shaft.