A bump for Keltos: PLANS!!!
Found some plans on this site:
http://www.rnsubs.co.uk/index.php
Specifically as part of this article:
http://www.rnsubs.co.uk/Boats/BR3043/contents.php
Covers pre WW1 A Class all the way up to the Porpoise class minelayers. The plans are reproduced to a large scale, just as well as they're a wee bit fuzzy!
Example - Group 1 S Class subs:
http://www.rnsubs.co.uk/Boats/BR3043/chapter16.php
Plan as built - by WW2 the retractable gun and superstructure had been removed and replaced by a conventional gun on the casing:
http://www.rnsubs.co.uk/Boats/BR3043...Plate%2041.jpg
HMS/m Sturgeon as built:
http://images-02.delcampe-static.net...96/106_001.jpg
As at the start of WW2:
http://www.secondworldwar.org.uk/subs/sturgeon1.jpg
Only four boats in this group and three had been sunk by the end of 1940!
Hope this helps!
Mike.