Lorne:
It was just happenstance or it came to me in a dream.

Here's a 221E action:



As Merkel1898- notes there was some method to their madness as well as rhyme or reason to their models. I'm not sure that the following applies to O/Us, which had models like 160(scattergun in both) & 167(combo), but the middle number of the model gives a hint at the tube types.

0 - scattergun
1 - combo
2 - twin rifled

Kind Regards,

Raimey