First cover, 11 screws, 1949 and partial 1950 Model 7 Oil over pressure vents above OPV into the timing chest.
T2004A early 11 screw    inner view T2004A early

Second cover, now with 12 screws which continues until the end of commando.
T2004A late 12 screw    inner view T2004A late

This cover has provision for a round Norton badge sept 55-sept 57 pn 19257
19257 outer early twins    inner view 19257

This cover is styled similar to the previous cover with the round badge, but it's internal functioning has changed dramatically. It has a fully enclosed internal oil pressure bypass. Due to the removeable to clean filter, there is no provision for upgrade to a high pressure rocker feed.
20040 outer 57-62 twins     20040 inner view 57-62 twins

Later 50's this cover allows the hook-up of a tach gearbox with a forward tilt of the cable.
T2004 tach drive early     inner view T2004

Last cover prior to commando, 650 and atlas 20M. Now with a round mounting surface for the tach mount. normally with a more upright cable but sometimes with a reoriented drilling for a forward tilt of the cable.
25186WM late tach drive     inner view 25186WM

First commando timing cover for 20M3. Still cam end tach drive.
However the biggest difference is the over pressure venting back into the engine block to the oil pump inlet.
25186AB1     inner view 25186AB1

This is a repeat of the 25186 cover but has a very small identifier difference. This cover was on a later 20M3 engine that had evolved to include a lowered timing chest drain.
25186AB2     inner view 25186AB2

First cover for the single chain family 20M3S + commando pn 06-1073 it has the ignition points and AA unit on the end of the cam.
061072 20M3S +     inner view 061072

Not a big change from prior cover but has a little boss below the OPV that may have been intended for the failed electric starter.
Still shown as PN 06-1073 Though the casting has changed.
063112 200000-300000    inner view 063112

MKIII commando cover, Timing chain check hole/plug, integral antisump check valve. Over pressure relief vents inside timing chest like first nortons. There may have been a few other engines that used this cover, specifically if the normal commando OPV flow passage was not fully drilled back to the oil pump input.
066161 MKIII    inner view 066161