Record Sheets: 3058 Update Unabridged (Inner Sphere)
Record Sheets: 3058 Update Unabridged (Clan & Star League)
Compared to the relevant portion of the old RS: 3058U units there are
53 new Combat Vees (though that includes some 21 or so RS: Upgrades and 1 relocated RS: 3039 vehicle)
25 new 'Mechs (though that includes the 8 corrected and re-canonised Dragon Roars configurations)
294 total RS
78 new units
Despite aiming to be an update document, 25% of this document is new. And more than 15% of this document is absolutely new, no matter which form.
The eras were all individually reviewed and co-ordinated with the MUL.
Every design was reviewed in good faith. Notice many errata, ranging from the subtle (the addition of CASE in the Lance Command Partisan) to the noticable (fixing the Pegasus to what will be its final form), up to the massive change that happened with the Clan Badger and Bandit.
Naming was standardised (and again co-ordinated with the MUL).
RS always are amongst the most error-prone products due to the gigantic amount of numbers faced by the product team. This product aims to minimise the margin of error and also bears the brunt of correcting many old ones.
Finally, returning to your original question. The Clan/SL/BA part of the release has
271 Record Sheets, including
49 new RS
And because I don't want to sound like a used car salesman, I'll put it the same you did before: 82% of the sheets were there in some form in the old RS: release.
You get 18% new, and all the above perks.