What does "M167" on the bottom of a 500 mean?

[Greg G.]

It can't be a manufacture date, they didn't have those feet in 67.  Or is it actually a refurb date, as in "Modified Jan 67"?  I think I answered my own question, I just noticed the tell-tale sign of a painted-over manufacture date just to the right.  Doesn't really look too modified though.

[paul-f]

The next step is to look at the dates on all the components and see if there's a cluster around the date on the network.  That is likely the assembly date.

If the changed parts are in 67, they were part of the refurb.

It also looks like there had been a screw in the empty hole on the left.  That's where a tube would have been mounted.  The original model number is painted over too.  It was probably converted from a 501.

[Dan/Panther]

I think it means it was originally a 1949 set, that got refurbed.
D/P

[paul-f]

During refurb, it looks like they replaced the housing, bottom plate and internals.   

[poplar1]

Isn't that like Grandpa's Axe?

You know the one where they changed the handle three times and the blade twice, but it is still Grandpa's axe?

[twocvbloke]

Isn't that like Grandpa's Axe?

You know the one where they changed the handle three times and the blade twice, but it is still Grandpa's axe?

Or Trigger's Broom, where in the Only fools and Horses episode, Heroes and Villains, Trigger wins an award for owning the same broom for 20 years, stating  that it has had 17 new heads and 14 new handles, but insists it is still the same broom...