Yes Juha, right you are, but initially the HP.52 and HP.53 were, on the drawing board at least, pretty much the same aeroplane, but yes, the original spec B.9/32 doesn't stipulate a torpedo carrying requirement.
The last time I was at Cosford I took some close up images of the rear position and the mods for the fitting of the torpedo, of the RAF Museum's example. There has been some confusion over the modifications made to the TB variant over the years - the step has often been regarded as evidence of the doors being altered, but they weren't; the rear gunner's position was reduced in depth and a prominent 'vee' was built into its underside to accommodate the torpedo with stabilising fins.
This image shows the Hampden TB.1's rear cupola and window layout, what isn't evident is the change in size compared to the standard bomber variant, but the windows are different.
View attachment 597923Hampden TB cupola i
This image shows the prominent cutout in the underside of the cupola showing the cutout for the torpedo, giving it reduced depth.
View attachment 597924Hampden TB cupola ii
This profile drawing shows the bomber variant profile, note the cupola design is different.
http://img.wp.scn.ru/camms/ar/1271/pics/9_1.jpg
This common image of a 489 (New Zealand) Sqn Hampden shows the prominent step that you mention, which is caused by the reduced size of the cupola, not an increase in the door size at their rear end.
ADF-Serials Image Gallery :: Handley Page Hampden :: HAMPDEN_AN127_455SQN_XA_Y