Starting with their January 2021 major website revision, Rega began quoting the following technical specifications for their tonearms.
Effective Mass - Low / 11 g
Mounting Distance (platter centre to arm hole centre) - 222 mm
Effective Length - 236.5 mm
Offset Angle - 21.55°
Overhang - 14.5 mm
According to my memory (and confirmed via web.archive.org), prior to that Rega had posted 17.2mm for overhang, this starting in 2017 with the introductions of RB220, RB330, and eventually RB880 (and subsequently purged sometime in February 2019 until such time as the new January 2021 site update rolled out, at which point all were listed as above).
Effective mass - Low / 11g
Overhang - 17.2mm
Mounting distance platter center to arm hole center - 222mm
From the above, we can deduce that effective length was considered to be 239.2, i.e. mounting distance plus overhang.
As an interesting aside, Origin Live quote the following for Rega tonearms. As OL were an official OEM reseller going back to RB250/300 days, some of this info will have come from Rega themselves.
https://www.originlive.com/faq-items/rega-tonearm-specifications/
Low effective mass - 11 grams
Effective length - 240 mm
Overhang - 17.2 mm
Offset angle - 23 degrees
Mounting distance: platter centre to arm hole centre - 221.7 mm
Note that effective length minus overhang should equal mounting distance. In OL's case 240 - 17.2 = 222.8mm which isn't what they state.
Given the above data, I suspect one of two things; 1) Rega redid their maths and came up with slightly different results (unlikely), or 2) starting with the introductions of RB220/330, Rega's latest tonearm tube castings include some slight geometric modifications relative to previous, in particular offset (likely).
