Refers to tubular glass, 10 mm diameter, according to this forum post.
The W5W defines the bulb "category", which is the standardized filament lamp design, as explained in ECE Regulation 37.
There is no explicit mention of the first "W" referring to "Wedge", but all bulb categories that start with a "W" share the same wedge-style base in the linked document.
Incidentally, W3W and W5W have the same dimensions, the former rated for 3 W output, the former rated for 5 W.
I couldn't find any reference to 501, but the bulb "type" is plausible.
The document distinguishes between "category" and "type":
2.1.1. Definition of "category"
The term "category" is used in this Regulation to describe different basic design of standardised filament
lamps. Each category has a specific designation, as for example: "H4",
"P21W", "T4W", "PY21W"or "RR10W".
2.1.2. Definition of "type"
Filament lamps of different "types" are filament lamps within the same category which differ in such essential respects as:
18.104.22.168. Trade name or mark (Filament lamps bearing the same trade name or mark but produced by different manufacturers are considered as
being of different types. Filament lamps produced by the same
manufacturer differing only by the trade name or mark may be
considered to be of the same type);
22.214.171.124. Bulb design and/or cap design, in so far as these differences affect the optical results;
126.96.36.199. Rated voltage;