In Program Manager Technical Advice PMTA 2018-008 the IRS discusses what are acceptable signatures for the business versions of Form 8453 (such as Form 8453C, 8453S, etc.).

In Notice 2007-79 the IRS allowed electronic return originators (EROs) to sign Form 8453, Form 8878, U.S. Individual Income Tax Declaration for an IRS e-file Return, Form 8878, IRS e-file Signature Authorization for Form 4868 or Form 2350; and Form 8879, IRS e-file Signature Authorization by specified alternative means. These means were:

Rubber stamp,
Mechanical device (such as signature pen), or
Computer software program.

The notice did contain the following caveat:

This notice applies only to EROs that sign Form 8453, Form 8878, or Form 8879, and does not alter the signature requirements for any other type of document currently required to be manually signed, such as elections, applications for changes in accounting method, powers of attorney, or consent forms. In addition, this notice does not alter the requirement that Form 8453, Form 8878, or Form 8879 be signed by the taxpayer making these forms by handwritten signature or other authorized means.

Rev. Proc. 2005-39 extended the same option to specified payroll tax forms.

However, the PMTA points out that this authority was never extended to cover the business versions of the Form 8453 series, nor did the payroll tax ruling cover Letter 3083, Returned Statement of Receipt (SoR) for the 94x On-Line Signature PIN.

IRC §6061(b)(1) provides the following:

(b) Electronic signatures

(1) In general

The Secretary shall develop procedures for the acceptance of signatures in digital or other electronic form. Until such time as such procedures are in place, the Secretary may—

(A) waive the requirement of a signature for; or

(B) provide for alternative methods of signing or subscribing,

a particular type or class of return, declaration, statement, or other document required or permitted to be made or written under internal revenue laws and regulations.

The PMTA notes that while the IRS had a revenue procedure project in its 2015 and 2016 Published Guidance Plans to deal with alternative signature options for all forms, no revenue procedure was ever issued. The project is no longer listed in the current Published Guidance Plans. The PMTA concludes that since the provision only allows the use of methods authorized by the IRS in the interim that:

Unless specifically provided for in published guidance, signatures on Service documents must be manually signed. Electronic signatures on Service documents are legally valid only if specifically allowed in published guidance.

Specifically, with regard to these business forms the PMTA concludes that:

As there is no published guidance authorizing the signing of the Form 8453 series for business returns and Letter 3083 by alternative of electronic means, these documents must be manually signed.